A system is provided for managing mobility of an Access Terminal (AT) in a Proxy Mobile Internet Protocol (PMIP) communication system. As the AT performs a handoff, a target Signaling Radio Network Controller (SRNC) delivers session information of the AT, acquired from a source SRNC of a source network, to a target Access Network (AN). An Authentication, Authorization and Accounting server (AAA) performs an AT access authentication process. Then, a target Access Gateway (AGW) performs a proxy registration procedure, with the target AN, and receives, from the AT, a message indicating a need to establish an interface with the HA. A Home Agent (HA) performs mobility management for the AT, receives a message for requesting proxy registration for mobility management for the AT, sends an access request message to the AAA, receives an access accept message, and sends a response message for proxy registration to the target AGW.