I am studying PLAI, Now I have already run chapter 3's code in DrRacket (with #lang plai), but I am a newbie in DrRacket. Can I edit and run the code only in Emacs?
The #lang plai directive will work in ordinary Racket files outside of DrRacket. There should be a racket executable in your installation of Racket that you can use to run these. Try geiser if you want an emacs package that makes it easier to run Racket programs. Quack is also helpful.
More generally, see the guide entry on using emacs with Racket. You might also want to start reading the guide from the beginning where it starts out by explaining how to run Racket.