Aiming to the fact that the distribution modules of the C3I simulation system originally developed based on TCP/IP protocol has to be compatible with the present HLA (High Level of Architecture) rules, three methods for implementing the interaction of HLA and TCP/IP subsystems are presented to make the original TCP/IP modules directly used in the HLA-based C3I system without redevelopment. The three methods are agent method, middleware method and gateway method. Moreover, the implementation of gateway method via VC++ as well as the key technology is discussed.