diff --git a/backend/app/services/financial_sync.py b/backend/app/services/financial_sync.py
index 6655496..6562bf4 100644
--- a/backend/app/services/financial_sync.py
+++ b/backend/app/services/financial_sync.py
@@ -160,7 +160,8 @@ def _merge_report_history(*frames: pl.DataFrame) -> pl.DataFrame:
     语义(区分"覆盖"与"填空"): 每列独立取 announce_date 最新的非空值 —
     新同步行有值则覆盖旧值, 新行缺的列(如 fuyao 不提供的字段)由旧行补齐,
     实现多数据源并集共存。历史报告期不可变, 合并不会引入过期数据。
-    无 announce_date 的帧按输入顺序, 后写优先(与旧行为 keep="last" 一致)。
+    无 announce_date 的帧按输入顺序, 后写优先(与旧行为 keep="last" 一致);
+    公告日为空视为最旧, 不得压过带公告日的行。
     """
     valid = [
         frame
@@ -176,7 +177,10 @@ def _merge_report_history(*frames: pl.DataFrame) -> pl.DataFrame:
     sort_keys = ["symbol", "period_end"] + (
         ["announce_date"] if "announce_date" in merged.columns else []
     )
-    merged = merged.sort(sort_keys, nulls_last=True)
+    # 公告日为空排在最前: 排到最后会让"公告日未知"的旧行在逐列 last() 时胜出,
+    # 产出 announce_date 是新公告、数值却是旧值的自相矛盾行。symbol/period_end
+    # 已在上面过滤掉空值, 不受该参数影响。
+    merged = merged.sort(sort_keys, nulls_last=False)
     value_cols = [c for c in merged.columns if c not in ("symbol", "period_end")]
     return (
         merged.group_by("symbol", "period_end")
diff --git a/backend/tests/test_fundamental_factors.py b/backend/tests/test_fundamental_factors.py
index c148678..9e4bc8f 100644
--- a/backend/tests/test_fundamental_factors.py
+++ b/backend/tests/test_fundamental_factors.py
@@ -210,3 +210,41 @@ def test_financial_sync_merges_history(tmp_path: Path):
     assert row.height == 1
     assert row["roe"].item() == 9.5
     assert merged2.height == 2  # 修正不增加行数
+
+
+def test_financial_sync_merge_unknown_announce_date_does_not_win():
+    """公告日未知的旧行不得压过带公告日的新行。
+
+    多源并存时旧行可能没有 announce_date (provider 不提供 / 上游缺该字段)。
+    这类"公告日未知"的行若排在真实公告日之后, 逐列取最后一个非空值时反而胜出,
+    合并结果会出现「announce_date 是新公告、数值仍是旧值」的自相矛盾行,
+    点时因子据此在公告日之后放出的是修正前的数。
+    """
+    from app.services import financial_sync as fs
+
+    revised = pl.DataFrame({
+        "symbol": ["600000.SH"],
+        "period_end": ["2025-12-31"],
+        "announce_date": ["2026-02-01"],
+        "roe": [9.5],
+    })
+    # 旧行有 announce_date 列但取值为空
+    old_null = pl.DataFrame({
+        "symbol": ["600000.SH"],
+        "period_end": ["2025-12-31"],
+        "announce_date": [None],
+        "roe": [9.0],
+    })
+    row = fs._merge_report_history(old_null, revised).to_dicts()[0]
+    assert row["announce_date"] == "2026-02-01"
+    assert row["roe"] == 9.5
+
+    # 旧帧整列缺失 (另一数据源不提供该字段) — 文档承诺"后写优先"
+    old_missing = pl.DataFrame({
+        "symbol": ["600000.SH"],
+        "period_end": ["2025-12-31"],
+        "roe": [9.0],
+    })
+    row = fs._merge_report_history(old_missing, revised).to_dicts()[0]
+    assert row["announce_date"] == "2026-02-01"
+    assert row["roe"] == 9.5