Abstract: Introducing briefly the concepts of NGN (Next Generation Network) and softswitch, and according to China's General Technical Requirements for Softswitch Devices, the design model based on H.248 protocol for IP user media gateways is presented basically. An emulator was designed to emulate the system so as to explain the function and implementations of each and every module and the voice handling flow between two user media gateways. The difficulties and key technique are discussed for system implementation. Using the design model can protect the huge investments in PSTN (Public Switched Telephone Network), resolve all existing problems of how to smoothen users' accessing NGN, implement many basic services especially the value-added services and enable PSTN to transit to NGN smoothly.