diff --git a/qlib/utils/__init__.py b/qlib/utils/__init__.py
index 2a94ebd555b..0f4d7b10876 100644
--- a/qlib/utils/__init__.py
+++ b/qlib/utils/__init__.py
@@ -932,7 +932,10 @@ def fname_to_code(fname: str):
 
     prefix = "_qlib_"
     if fname.startswith(prefix):
-        fname = fname.lstrip(prefix)
+        # NOTE: use slicing rather than ``lstrip(prefix)``; ``str.lstrip`` strips
+        # any leading characters contained in ``prefix`` (e.g. it would turn
+        # "_qlib_lpt1" into "pt1"), not the prefix as a whole.
+        fname = fname[len(prefix) :]
     return fname
 
 
diff --git a/tests/misc/test_utils.py b/tests/misc/test_utils.py
index db5b0724886..0b0a01c2573 100644
--- a/tests/misc/test_utils.py
+++ b/tests/misc/test_utils.py
@@ -10,6 +10,7 @@
 from qlib.constant import REG_CN, REG_US, REG_TW
 from qlib.utils.time import cal_sam_minute as cal_sam_minute_new, get_min_cal, CN_TIME, US_TIME, TW_TIME
 from qlib.utils.data import guess_horizon
+from qlib.utils import code_to_fname, fname_to_code
 
 REG_MAP = {REG_CN: CN_TIME, REG_US: US_TIME, REG_TW: TW_TIME}
 
@@ -127,6 +128,22 @@ def test_guess_horizon(self):
         result = guess_horizon(label)
         assert result == 5
 
+
+class FileNameUtils(TestCase):
+    def test_fname_code_round_trip(self):
+        # code_to_fname only prefixes reserved Windows device names; fname_to_code
+        # must strip the whole "_qlib_" prefix, not individual characters.
+        # exists_qlib_data() lowercases the directory name before converting back,
+        # and "lpt*" begins with characters that are also in the prefix.
+        for code in ["CON", "PRN", "AUX", "NUL", "COM1", "LPT1", "LPT9"]:
+            fname = code_to_fname(code)
+            self.assertEqual(fname_to_code(fname.lower()), code.lower())
+
+        # a name whose body consists only of prefix characters must survive
+        self.assertEqual(fname_to_code("_qlib_lll"), "lll")
+        # plain codes pass through unchanged
+        self.assertEqual(fname_to_code("AAPL"), "AAPL")
+
         label = ["Ref($close, -1) / Ref($close, -1) - 1"]
         result = guess_horizon(label)
         assert result == 1