At present, using Mobile IP for complementing the link layer mobility in 3G has been researched widely, although MIP is not be supported explicitly in 3GPP. In this paper, we discuss the necessity of deployment MIPv6 in 3GPP. Furthermore, IMS (IP Multimedia Subsystem) was deployed in 3GPP. IMS uses SIP protocol to manage the multimedia sessions. In order to seamlessly support the real-time multimedia communication in mobility environment, it is important to interwork SIP and MIP. Some problems about interworking SIP and MIP are analysed. Then an optimized solution of interworking between SIP and MIPv6 is presented. The solution provides the effective routing and reduces the handoff delay and traffic burden inside Home Agent.