diff --git a/csst_common/io.py b/csst_common/io.py
index edb2369f12127d0c9eadb715ea4af0a033b56900..5379d0a9173cb6cd6dcb82573674810b86625955 100644
--- a/csst_common/io.py
+++ b/csst_common/io.py
@@ -6,10 +6,12 @@ Author:         Bo Zhang
 Created:        2023-12-13
 Modified-History:
     2023-12-10, Bo Zhang, created
-    2023-12-15, Bo Zhang, add verify_checksum and append_header, add module header
+    2023-12-15, Bo Zhang, added verify_checksum and append_header, add module header
+    2023-12-16, Bo Zhang, fixed a bug in finding the first key when reformatting
 """
 import warnings
 from copy import deepcopy
+from typing import Optional
 
 from astropy.io import fits
 
@@ -86,7 +88,34 @@ def append_header(
     return original_h
 
 
-def reformat_header(h: fits.Header, strip=True, comment=None):
+def reformat_header(
+    h: fits.Header,
+    strip: bool = True,
+    comment: Optional[str] = None,
+):
+    """
+    Reformat fits Header.
+
+    Remove blanks, comments, and history, and add a new comment at the beginning.
+
+    Parameters
+    ----------
+    h : fits.Header
+        The original header.
+    strip: bool = True
+        If `True`, strip cards like SIMPLE, BITPIX, etc.
+        so the rest of the header can be used to reconstruct another kind of header.
+    comment: Optional[str] = None
+        The new comment at the beginning.
+
+    See Also
+    --------
+    astropy.io.fits.Header.strip()
+
+    References
+    ----------
+    https://docs.astropy.org/en/stable/io/fits/api/headers.html#astropy.io.fits.Header.strip
+    """
     h_copy = deepcopy(h)
     # strip
     if strip: