I can't export my file into "atlas.txt" which always just .atlas file
To export your file with the ".atlas.txt" extension instead of just ".atlas", you need to adjust the export settings in the Spine Editor. Under the "Export" section, go to "Pack Settings" and ensure that the atlas extension is set to ".atlas.txt". This is particularly important if you are using Unity, as it requires the ".atlas.txt" extension for the runtime. If you are still encountering issues, double-check that the settings are correctly applied before exporting.
Then I open that file which will show atlas.txt at the end of name. So is it alright if I import that file into Unity? 
VictorTran 
你应该打开这个，你的文件已经是.txt结尾的了
@VictorTran Your files are fine. Please note that you've just hidden file extensions, hiding anything starting at the last . character. So Spine_Gacha.png is displayed as Spine_Gacha and Spine_Gacha.atlas.txt as Spine_Gacha.atlas.
Thank everyone a lot!