How can I turn off the prompt when I try to save the file in Emacs?
I mean every time I do C-xs, emacs always ask "Save file xxx?"
How can I disable this prompt?
You're looking for the command C-x C-s (keep holding control when you hit the "s"). That doesn't prompt before save.
C-x s
(what you had been using) attempts to save all files and buffers while ivarley's recommendation is just for the current file/buffer.