A communication method and system for terminal entering and leaving idle mode in a mobile communication system are disclosed. The communication method for a terminal entering an idle mode includes: triggering a flow of entering the idle mode for the terminal; and a serving access service network gateway sending the information of anchor function entities serving the terminal to an anchor function entity information storing unit, so as to store the information. With the technical scheme of the invention, when the anchor function entities serving the terminal are not located at the same physical entity, the addressing process can be accomplished correctly, and the smoothness of the network service flow can be guaranteed.