Time synchronization is the basis of network real-time communication. The combination of IP Multimedia Subsystem (IMS) and Overlay Networks is an important direction of the development of IP communication. Facing the needs and issues of IMS overlay network architecture, this paper proposes a new time synchronization mechanism that fits well in IMS overlay network and completes the protocol definition. The behavior of modifying the expansion of open source software SIPp achieves the function of time synchronization service using the transmission of the SIP message to accomplish, and verifies the effectiveness of the proposed scheme.