Thanks Wolf-Dieter. I had resolved it lastnight.
The method is copy the EJB interface class into the Web App's class path. and then the Web App can find the EJB class.
And I also test the other method, I add the EJB remote interface (UsersDaoRemote.java) into the Web App's project, it work fine too!
I had confused about the Web Container and the EJB container, since the Web Container have more and more functions about EJB(likes WebBean). Does it have possible that the Web Container replace the EJB Container, or the two Container combine to one Container in the future?