I'm sure that the file is in .amule/Incoming directory, but I cannot delete the Incoming directory by 'rm -rf Incoming' command. And I can handle the INVISIBLE file by the name shown in amulegui with programs such as file, cp, mv. Not all programs can handle it, for example, md5sum, ls.
So I move the file to another place. The file is still invisible, but I can list it in flashfxp (in winxp), proftpd as ftp server.
Now I can copy this file by 'downloading', hope I can remove it to recycle disk space.
Thank you all.