Thank you for your reply.
I have tried the methods you suggested, but the time colume of the readings from the spreadsheet was all round up (like 0,10,10,10,20,20,20,20,30ms...rather than 0,1,2,3,4ms,...).
If I save this .plw file as .txt file, the time colume of the readings would be the same in the .txt file.
I think that the format of the data in the .plw file may be altered in the the spreadsheet.
So I want to convert the original .plw file directly into .txt file(uncode), which would give the correct time colume(0,1,2,3,4ms,...). Is there a possible way to do that.