diff --git a/sanguo_api/routes_live.py b/sanguo_api/routes_live.py
index 5042cfb..cf8dec9 100644
--- a/sanguo_api/routes_live.py
+++ b/sanguo_api/routes_live.py
@@ -153,6 +153,8 @@ def create_live(req: LiveCreateRequest):
         payload["mini_path"] = (
             os.environ.get("SANGUO_QMT_PATH") or _DEFAULT_MINI_PATH
         )
+    # B3 预算硬限制:Σ预算 ≤ 账户现金(fail-closed,快照不可用拒绝创建)
+    _enforce_budget(db, payload["account"], payload["initial_capital"])
     aid = save_account(db, {**payload, "status": "stopped"})
     return {"account_id": aid, "status": "stopped"}
 
@@ -192,6 +194,70 @@ def list_lives():
     return {"accounts": items}
 
 
+# ===== B3 预算硬限制(spec §multi-strategy-instance-budget §B3) =====
+
+def _budget_state(
+    db: str, account: str, exclude_aid: int | None = None
+) -> dict:
+    """预算占用状态:剩余 = 账户现金 − Σ 同账户其他实盘实例预算。
+
+    fail-closed:快照缺失/过期(>10min)→ fresh=False,remaining=None(不猜数)。
+    Σ 不分运行/停止——停止的实例持仓仍占着账户资金。
+    """
+    from sanguo_live.persistence import (
+        get_fresh_account_snapshot, list_accounts,
+    )
+
+    acc = (account or "").strip()
+    allocated = sum(
+        float(r.get("initial_capital") or 0)
+        for r in list_accounts(db)
+        if (r.get("account") or "").strip() == acc
+        and r.get("id") != exclude_aid
+    )
+    snap = get_fresh_account_snapshot(db, acc) if acc else None
+    if snap is None:
+        return {"account": acc, "fresh": False, "account_cash": None,
+                "allocated": allocated, "remaining": None,
+                "snapshot_at": None}
+    cash = float(snap.get("cash") or 0)
+    return {"account": acc, "fresh": True, "account_cash": cash,
+            "allocated": allocated, "remaining": cash - allocated,
+            "snapshot_at": snap.get("updated_at")}
+
+
+def _enforce_budget(
+    db: str, account: str, initial_capital: float,
+    exclude_aid: int | None = None,
+) -> dict:
+    """新建/修改实盘的预算校验:超限或快照不可用 → 400(报文带剩余数)。"""
+    st = _budget_state(db, account, exclude_aid)
+    if not st["fresh"]:
+        raise HTTPException(
+            400, f"账户 {account} 快照不可用(未采集或超过10分钟),稍后再试")
+    if float(initial_capital) > st["remaining"]:
+        raise HTTPException(
+            400,
+            f"预算超限:账户可用现金 {st['account_cash']:.0f},"
+            f"已分配 {st['allocated']:.0f},剩余可分配 {st['remaining']:.0f}"
+            f"(本次 {float(initial_capital):.0f})")
+    return st
+
+
+@router.get("/live/budget-info", dependencies=[Depends(verify_token)])
+def get_budget_info(account: str = ""):
+    """预算信息(前端表单默认值=remaining + 占用率条)。
+
+    注意:必须注册在 /live/{aid} 之前(FastAPI 路径匹配不按类型分流,
+    否则 budget-info 会被 {aid} 吃掉 422)。快照不新鲜时 fresh=False、
+    remaining=None——前端禁用提交并提示稍后再试(fail-closed 同款语义)。
+    """
+    db = _db_path["path"]
+    if not db:
+        raise HTTPException(400, "db 未配置")
+    return _budget_state(db, account)
+
+
 @router.get("/live/{aid}", dependencies=[Depends(verify_token)])
 def get_live(aid: int):
     from sanguo_live.persistence import get_account, get_first_balance, get_last_balance
@@ -290,6 +356,8 @@ class LiveUpdateRequest(BaseModel):
     strategy_name: str | None = None
     setting: dict | None = None
     interval: str | None = None
+    # B3:预算可改(initial_capital 含义升级为资金额度)
+    initial_capital: float | None = None
 
 
 @router.put("/live/{aid}", dependencies=[Depends(verify_token)])
@@ -305,6 +373,18 @@ def update_live(aid: int, req: LiveUpdateRequest):
         raise HTTPException(404, "account not found")
     if acc["status"] == "running":
         raise HTTPException(400, "运行中不可编辑，请先停止实例")
+    # B3:预算或账号变更时校验(改名等不触发——不是预算事件,夜间 QMT 关闭
+    # 也能改名)。exclude 自身:自己的旧预算不重复计入 Σ。
+    if req.initial_capital is not None or (
+        req.account and req.account != acc["account"]
+    ):
+        _enforce_budget(
+            db,
+            req.account if req.account is not None else acc["account"],
+            req.initial_capital if req.initial_capital is not None
+            else float(acc.get("initial_capital") or 0),
+            exclude_aid=aid,
+        )
     sets, args = [], []
     field_map = {
         "name": req.name, "account": req.account, "vt_symbol": req.vt_symbol,
@@ -320,6 +400,8 @@ def update_live(aid: int, req: LiveUpdateRequest):
             sets.append(f"{col}=?"); args.append(v)
     if req.setting is not None:
         sets.append("setting=?"); args.append(json.dumps(req.setting))
+    if req.initial_capital is not None:
+        sets.append("initial_capital=?"); args.append(req.initial_capital)
     if not sets:
         return {"account_id": aid, "updated": False}
     with sqlite3.connect(db) as conn:
diff --git a/tests/api/test_instance_binding.py b/tests/api/test_instance_binding.py
index 358de2f..5211ee3 100644
--- a/tests/api/test_instance_binding.py
+++ b/tests/api/test_instance_binding.py
@@ -21,6 +21,11 @@ def _setup(tmp_path, monkeypatch):
     app = create_app(db_path=pdb)
     set_paper_db(pdb)
     set_live_db(ldb)
+    # B3 起 live/create 需新鲜账户快照,测试给用到的账号播种一份
+    from sanguo_live import persistence as _lp
+    for _acc in ("8886000519", "123"):
+        _lp.upsert_account_snapshot(ldb, _acc, cash=1e9, market_value=0,
+                                    total=1e9, positions=[])
     return TestClient(app), create_token("admin")
 
 
diff --git a/tests/api/test_live_budget.py b/tests/api/test_live_budget.py
new file mode 100644
index 0000000..4045de0
--- /dev/null
+++ b/tests/api/test_live_budget.py
@@ -0,0 +1,176 @@
+"""Tests for B3 预算硬限制(spec §B3):create/update 校验 + /live/budget-info。
+
+直调路由函数(与 test_portfolio_live.py 同风格,免 auth)。
+核心断言:剩余=现金−Σ其他实例预算;超限/快照过期 → 400 fail-closed。
+"""
+import sqlite3
+from datetime import datetime, timedelta, timezone
+
+import pytest
+from fastapi import HTTPException
+
+from sanguo_api import routes_live as rl
+from sanguo_live import persistence as lp
+
+
+@pytest.fixture()
+def live_db(tmp_path):
+    db = str(tmp_path / "live.db")
+    rl.set_db_path(db)
+    return db
+
+
+def _snapshot(db, account="66639661", cash=1_000_000.0, age_min=0):
+    lp.upsert_account_snapshot(
+        db, account, cash=cash, market_value=0.0, total=cash, positions=[])
+    if age_min:
+        old = (datetime.now(timezone.utc)
+               - timedelta(minutes=age_min)).isoformat()
+        with sqlite3.connect(db) as conn:
+            conn.execute(
+                "UPDATE qmt_account_snapshot SET updated_at=? WHERE account=?",
+                (old, account))
+
+
+def _create(db, capital=500_000.0, account="66639661"):
+    req = rl.LiveCreateRequest(
+        account=account, strategy_name="p1", strategy_type="portfolio",
+        strategy_class="all_weather", initial_capital=capital)
+    return rl.create_live(req)["account_id"]
+
+
+# ---------------- budget-info ----------------
+
+def test_budget_info_basic(live_db):
+    _snapshot(live_db, cash=1_000_000)
+    _create(live_db, capital=300_000)  # 已分配 30 万
+    st = rl.get_budget_info("66639661")
+    assert st["fresh"] is True
+    assert st["account_cash"] == 1_000_000
+    assert st["allocated"] == 300_000
+    assert st["remaining"] == 700_000
+
+
+def test_budget_info_includes_stopped(live_db):
+    """停止实例持仓仍占资金 → Σ 不分状态。"""
+    _snapshot(live_db, cash=1_000_000)
+    aid = _create(live_db, capital=300_000)
+    lp.update_account_status(live_db, aid, "stopped")
+    assert rl.get_budget_info("66639661")["allocated"] == 300_000
+
+
+def test_budget_info_stale_fail_closed(live_db):
+    _snapshot(live_db, cash=1_000_000, age_min=30)
+    st = rl.get_budget_info("66639661")
+    assert st["fresh"] is False
+    assert st["account_cash"] is None
+    assert st["remaining"] is None
+
+
+def test_budget_info_missing_snapshot(live_db):
+    st = rl.get_budget_info("NO_SUCH")
+    assert st["fresh"] is False
+    assert st["remaining"] is None
+
+
+# ---------------- create 校验 ----------------
+
+def test_create_within_budget_ok(live_db):
+    _snapshot(live_db, cash=1_000_000)
+    _create(live_db, capital=300_000)
+    _create(live_db, capital=700_000)  # 恰好用满
+    assert rl.get_budget_info("66639661")["remaining"] == 0
+
+
+def test_create_over_budget_400_with_remaining(live_db):
+    _snapshot(live_db, cash=1_000_000)
+    _create(live_db, capital=300_000)
+    with pytest.raises(HTTPException) as e:
+        _create(live_db, capital=800_000)
+    assert e.value.status_code == 400
+    assert "700000" in e.value.detail  # 报文带剩余数
+    assert "剩余可分配" in e.value.detail
+
+
+def test_create_snapshot_missing_400(live_db):
+    """快照不存在 → fail-closed(不猜数)。"""
+    with pytest.raises(HTTPException) as e:
+        _create(live_db)
+    assert e.value.status_code == 400
+    assert "快照不可用" in e.value.detail
+
+
+def test_create_snapshot_stale_400(live_db):
+    _snapshot(live_db, cash=1_000_000, age_min=30)
+    with pytest.raises(HTTPException) as e:
+        _create(live_db)
+    assert e.value.status_code == 400
+
+
+def test_create_budget_only_checks_same_qmt_account(live_db):
+    """Σ 只算同 QMT 账号——别的账号的实例不挤本账户池。"""
+    _snapshot(live_db, account="A1", cash=500_000)
+    _snapshot(live_db, account="A2", cash=900_000)
+    _create(live_db, account="A1", capital=400_000)
+    # A1 剩 10 万,但 A2 独立
+    assert rl.get_budget_info("A2")["remaining"] == 900_000
+
+
+# ---------------- update 校验 ----------------
+
+def _make_editable(live_db):
+    _snapshot(live_db, cash=1_000_000)  # create 需新鲜快照
+    return _create(live_db, capital=300_000)
+
+
+def _age_snapshot(db, account="66639661"):
+    old = (datetime.now(timezone.utc) - timedelta(minutes=30)).isoformat()
+    with sqlite3.connect(db) as conn:
+        conn.execute(
+            "UPDATE qmt_account_snapshot SET updated_at=? WHERE account=?",
+            (old, account))
+
+
+def test_update_budget_over_400_excludes_self(live_db):
+    """改预算超限 → 400;Σ 不含自身旧值(300 万自身不重复计)。"""
+    aid = _make_editable(live_db)  # 快照 100万
+    # 另一实例占了 50 万 → 剩余 50 万;自己 30 万不计入
+    _create(live_db, capital=500_000)
+    with pytest.raises(HTTPException) as e:
+        rl.update_live(aid, rl.LiveUpdateRequest(initial_capital=600_000))
+    assert e.value.status_code == 400
+    assert "500000" in e.value.detail
+
+
+def test_update_budget_within_ok(live_db):
+    aid = _make_editable(live_db)
+    _create(live_db, capital=500_000)
+    rl.update_live(aid, rl.LiveUpdateRequest(initial_capital=500_000))
+    acc = lp.get_account(live_db, aid)
+    assert acc["initial_capital"] == 500_000
+
+
+def test_update_rename_skips_budget_check(live_db):
+    """仅改名不触发校验——快照过期也能改名(不是预算事件)。"""
+    aid = _make_editable(live_db)
+    _age_snapshot(live_db)
+    rl.update_live(aid, rl.LiveUpdateRequest(name="new_name"))
+    assert lp.get_account(live_db, aid)["name"] == "new_name"
+
+
+def test_update_budget_change_blocked_when_stale(live_db):
+    """快照过期时改预算 → 400(改预算必须看到新鲜现金)。"""
+    aid = _make_editable(live_db)
+    _age_snapshot(live_db)
+    with pytest.raises(HTTPException) as e:
+        rl.update_live(aid, rl.LiveUpdateRequest(initial_capital=100_000))
+    assert e.value.status_code == 400
+    assert "快照不可用" in e.value.detail
+
+
+def test_delete_frees_budget(live_db):
+    """删除实例释放预算(重建流程依赖)。"""
+    _snapshot(live_db, cash=1_000_000)
+    aid = _create(live_db, capital=300_000)
+    rl.delete_live(aid)
+    assert rl.get_budget_info("66639661")["remaining"] == 1_000_000
diff --git a/tests/api/test_portfolio_live.py b/tests/api/test_portfolio_live.py
index 98b9ab2..400cde2 100644
--- a/tests/api/test_portfolio_live.py
+++ b/tests/api/test_portfolio_live.py
@@ -17,6 +17,10 @@ from sanguo_live import runner as live_runner
 def live_db(tmp_path):
     db = str(tmp_path / "live.db")
     rl.set_db_path(db)
+    # B3 起 create 校验预算(需新鲜账户快照),测试播种一份
+    live_persistence.upsert_account_snapshot(
+        db, "66639661", cash=1_000_000_000.0, market_value=0.0,
+        total=1_000_000_000.0, positions=[])
     return db
 
 
@@ -264,6 +268,9 @@ def test_update_live_normalizes_vt_symbol(tmp_path, monkeypatch):
     db = os.path.join(str(tmp_path), "l.db")
     app = create_app(db_path=db)
     set_db_path(db)
+    from sanguo_live import persistence as _lp
+    _lp.upsert_account_snapshot(db, "A1", cash=1e9, market_value=0,
+                                total=1e9, positions=[])  # B3 预算校验前置
     c = TestClient(app)
     h = {"Authorization": f"Bearer {create_token('admin')}"}