According to the current situation of CI system today, a three-layer modal of communication network including Link Maintenance Layer, Network Transmission Layer and Information Expression Layer is presented in this paper based on ISO/OSI reference modal. Then the Protocol in each layer has been analyzed and the design method has been put forward accordingly. Especially the function and realization method of Network Transmission Layer has been discussed and the route selection protocol applicable to CI system communication network for air defense has been presented.