A method and system for supporting a handoff of an MS in a PMIP mobile communication system are provided. The method includes a target node transmitting a first registration request message to an HA in order to request registration of the MS, when the MS moves from the service area of a serving node to the service area of the target node; the serving node discarding a second registration request message without processing the second registration request message, when the serving node receives the second registration request message from the MS before the target node receives a first registration response message for the first registration request message from the HA; and the serving node requesting deregistration of the serving node to the HA, when the target node receives the first registration response message from the HA.