diff --git a/backend/app/services/preferences.py b/backend/app/services/preferences.py
index bc9b9a2..4275493 100644
--- a/backend/app/services/preferences.py
+++ b/backend/app/services/preferences.py
@@ -88,13 +88,12 @@ def get_realtime_quote_interval() -> float:
 
 
 def set_realtime_quote_interval(interval: float) -> float:
-    """保存行情轮询间隔（不在此做 min/max 校验，由调用方按档位限制）。"""
-    current = load()
-    current["realtime_quote_interval"] = interval
-    _path().write_text(
-        json.dumps(current, indent=2, ensure_ascii=False), encoding="utf-8",
-    )
-    _invalidate_cache()
+    """保存行情轮询间隔（不在此做 min/max 校验，由调用方按档位限制）。
+
+    走 save() 而不是自己 load + write_text: 锁外的 read-modify-write 会用旧快照
+    整体覆盖文件, 把并发写入的另一个偏好丢掉 (见 save 的 docstring)。
+    """
+    save({"realtime_quote_interval": interval})
     return interval
 
 
diff --git a/backend/tests/test_preferences_concurrent_write.py b/backend/tests/test_preferences_concurrent_write.py
new file mode 100644
index 0000000..15c685e
--- /dev/null
+++ b/backend/tests/test_preferences_concurrent_write.py
@@ -0,0 +1,104 @@
+"""并发写 preferences.json 不得互相覆盖。
+
+preferences.save 的 docstring 记着这个坑: "FastAPI 同步端点跑线程池, 并行 PUT
+各自基于旧快照写盘会互相覆盖", 所以 save 的 read-modify-write 整段在 _SAVE_LOCK
+里。set_realtime_quote_interval 是唯一一个绕开该锁、自己 load + write_text 的
+setter —— PUT /api/settings/preferences/quote-interval 与任意另一个偏好 PUT
+同时在飞时, 后者会被前者用旧快照整体覆盖掉。
+"""
+from __future__ import annotations
+
+import json
+import threading
+
+import pytest
+
+from app.services import preferences
+
+_OTHER_KEY = "realtime_quotes_enabled"
+
+
+@pytest.fixture
+def prefs_path(tmp_path, monkeypatch):
+    path = tmp_path / "preferences.json"
+    monkeypatch.setattr(preferences, "_path", lambda: path)
+    preferences._invalidate_cache()
+    yield path
+    preferences._invalidate_cache()
+
+
+def _read(path) -> dict:
+    return json.loads(path.read_text(encoding="utf-8"))
+
+
+def test_interval_setter_does_not_clobber_a_concurrent_save(prefs_path, monkeypatch):
+    """轮询间隔写入与另一个偏好写入并发时, 两个键都要留下。
+
+    用一个会在第一次调用时挂起的 load 替身制造交错: 间隔 setter 拿到快照后
+    停住, 另一个 save 完整跑完, 然后间隔 setter 继续写盘。
+    """
+    preferences.save({_OTHER_KEY: False})
+
+    first_load_entered = threading.Event()
+    release_first_load = threading.Event()
+    other_save_done = threading.Event()
+    load_calls = []
+    real_load = preferences.load
+
+    def _load_pausing_on_first_call() -> dict:
+        snapshot = real_load()
+        load_calls.append(1)
+        if len(load_calls) == 1:
+            first_load_entered.set()
+            release_first_load.wait(10)
+        return snapshot
+
+    monkeypatch.setattr(preferences, "load", _load_pausing_on_first_call)
+
+    def _set_interval() -> None:
+        preferences.set_realtime_quote_interval(9.0)
+
+    def _save_other() -> None:
+        preferences.save({_OTHER_KEY: True})
+        other_save_done.set()
+
+    interval_thread = threading.Thread(target=_set_interval, name="set-interval")
+    interval_thread.start()
+    assert first_load_entered.wait(10), "间隔 setter 没有进入 load"
+
+    other_thread = threading.Thread(target=_save_other, name="save-other")
+    other_thread.start()
+    # 有锁时另一个 save 会一直等到间隔 setter 写完 (这里超时是预期的);
+    # 无锁时它会立刻写完, 随后被间隔 setter 的旧快照覆盖。
+    other_save_done.wait(0.5)
+    release_first_load.set()
+
+    interval_thread.join(10)
+    other_thread.join(10)
+    assert not interval_thread.is_alive() and not other_thread.is_alive()
+
+    monkeypatch.setattr(preferences, "load", real_load)
+    preferences._invalidate_cache()
+    stored = _read(prefs_path)
+    assert stored["realtime_quote_interval"] == 9.0
+    assert stored[_OTHER_KEY] is True, "并发的偏好写入被间隔 setter 的旧快照覆盖了"
+
+
+def test_interval_setter_keeps_existing_keys(prefs_path):
+    """顺序场景: 写间隔不能丢掉文件里已有的其它偏好。"""
+    preferences.save({_OTHER_KEY: True, "minute_sync_enabled": True})
+
+    preferences.set_realtime_quote_interval(3.0)
+
+    stored = _read(prefs_path)
+    assert stored == {
+        _OTHER_KEY: True,
+        "minute_sync_enabled": True,
+        "realtime_quote_interval": 3.0,
+    }
+
+
+def test_interval_setter_returns_value_and_refreshes_cache(prefs_path):
+    """返回值与缓存失效行为保持不变。"""
+    assert preferences.set_realtime_quote_interval(12.5) == 12.5
+    assert preferences.get_realtime_quote_interval() == 12.5