Referring to IN/Internet interworking models proposed by ITU-T (International Telecommunication Union-Telecommunication standardization sector) and IETF (the Internet Engineering Task Force) respectively, a new model is designed based on CMIN02 (China Mobile Intelligent Network version 02) system of BUPT (Beijing University of Posts and Telecommunications). This model carried out heavy modifications and refinements on the former two reference models. This model can be divided into three layers horizontally and three sections vertically. Because of the importance of gateways, emphases are put on them especially. Moreover, a novel notion called information flow trail is proposed, which is useful to design complex gateways. Most of previous works on gateways are devoted to studying simple cases where gateways have only two interfaces towards outside. On the contrary, MGF (Management Gateway Function), SCGF (Service Control Gateway Function) and C/BGF (Call/Bearer Gateway Function) have more than two interfaces and the novel notion is useful. During the implementation of the project 863 the modified model has been tested and proved viable.