After translation, I use File > Save Target As and Batch Task > Export, but the content in the target file the same as that of the source file.
No errors prompt during the export.

After translation, I use File > Save Target As and Batch Task > Export, but the content in the target file the same as that of the source file.
No errors prompt during the export.
Alas, sparse is the information you provide, but the normal way to export your target translations is to use the "Finalise" batch task (if you want your main TMs to be updated) or the "Generate target translations" batch task (if you just want the files but not update the main TMs - maybe because it's not signed off yet).