RigidBody doesn't sleep with gravity turned on.

To save CPU, rigidbody would go to sleep when velocity and angural velocity become low. However, I find that rigidbody won’t go sleep if the feild “use gravity” is on. Even if the rigidbody has stopped on the floor.

How can I solve this problem?

I had a question about collider penetration a few month ago where I get the tip that a small “Min Penetration for Penalty Force” and/or a high iteration count will prevent the bodies from sleeping. May be that will also help you: http://answers.unity3d.com/questions/399064/prevent-collider-intersection.html