Reason: It is caused by the template file of word.

1. First, open the start menu bar in the lower left corner of the computer desktop and select "Run".

2. Then click and enter "%appdata%\microsoft\templates" in the pop-up running window, and press enter to confirm.

3. Then find the "Normal.dotm" template file in the pop-up window, right-click and select "Delete".

Then you can solve this problem, and opening the Word document will not change the format.