diff --git a/sanguo_factor/monthly_batch.py b/sanguo_factor/monthly_batch.py
index d62ec6ba..0a73b96a 100644
--- a/sanguo_factor/monthly_batch.py
+++ b/sanguo_factor/monthly_batch.py
@@ -37,8 +37,8 @@ def month_end(as_of: str) -> str:
 def window_start(end: str, window_months: int = 12) -> str:
     """闭区间起点:含 end 当月共 window_months 个月的月初(起步默认 12M)."""
     y, m = int(end[:4]), int(end[5:7])
-    total = y * 12 + m - (window_months - 1)
-    return f"{total // 12:04d}-{total % 12:02d}-01"
+    total = y * 12 + (m - 1) - (window_months - 1)
+    return f"{total // 12:04d}-{total % 12 + 1:02d}-01"
 
 
 def resolve_with_yaml(registry: dict):
diff --git a/tests/factor/test_monthly_batch.py b/tests/factor/test_monthly_batch.py
index 949c3dac..ba4bbb20 100644
--- a/tests/factor/test_monthly_batch.py
+++ b/tests/factor/test_monthly_batch.py
@@ -44,6 +44,15 @@ def test_window_start_twelve_months():
     assert window_start("2026-09-30", 12) == "2025-10-01"
 
 
+def test_window_start_december_month_math():
+    """11 月月末回归锚:1-based 月减法 %12 归零曾产 '2026-00-01'(月=00 非法)."""
+    from sanguo_factor.monthly_batch import window_start
+    assert window_start("2026-11-30") == "2025-12-01"   # 含 11 月共 12 个月
+    assert window_start("2026-12-31") == "2026-01-01"
+    assert window_start("2026-01-31") == "2025-02-01"
+    assert window_start("2026-09-30") == "2025-10-01"   # 既有正确值不变
+
+
 def test_plan_batch_excludes_dead_and_labels(registry):
     plan = plan_batch(registry, "2026-09-30")
     assert plan["factor_names"] == ["vol_ma5"]