A method for processing resource, a communication system, and a mobility management network element are provided. The method includes: receiving, by a mobility management network element in a packet switched (PS) network, a Release Request message sent by an access network of the PS network or a Handoff Complete message sent by a circuit switched (CS) network when a user equipment (UE) is handed over from the PS network to the CS network; and processing, by the mobility management network element of the PS network, resources of the UE in the PS network. Thus, the processing of resources of the UE in the PS network is achieved when the UE is handed over from the PS network to the CS network. A communication system and a mobility management network element are also provided.