Abstract: The new generation telephone communication system of SGCC(State Grid Company of China) that based IMS technology has been completed until 2017, The external network users can access to IMS administrative switching network through the data communication network, by deploying mobile business data gateway, so then it can provide high-quality network users voice, conference and data services. In addition, IMS administrative switching network could provide a basic platform for other business systems by virtue of its open business architecture, and in consideration of the company's future wireless private network construction, IMS network is more easier to support all types of mobile users access and provide them much more multimedia communication services with its integrated technical characteristics,. Due to the openness of the wireless interface of mobile communication network, IMS mobile terminal users face different threats such as data eavesdropping, deleting, and tampering and so on. Hackers can even implement denial of service or block business by physical interference or protocol intervention. Therefore, according to the company's overall information security requirements, IMS internal and external network mobile business applications should follow the corresponding security requirements. Firstly it introduced the characteristics of IMS network architecture, then analyzed the risk and security requirements of IMS control domain, IMS media domain, IMS AS domain, IMS network management domain, user access domain, interconnection domain and HSS data domain, and it put forward the relevant security strategy. The IMS equipments used IP packet communication network to transport signal and media data flow, it deeply analysis the main attack types and attack principles of each layer (the equipment layer, the network layer and the application layer), and gave some solutions about security protection. Meanwhile, based on the IMS network architecture and the requirements of the company's network security system, it introduced the different mobile application requirements and terminal types, and put forward both the internal network and external network businesses interconnection methods about the mobile service application for mobile terminals or users. According to the security area of users, the different security solutions of the external network mobile service application were studied respectively. It gave four kinds of solutions according access mode, they are : special service mode, the SBC access mode, the callback method and the new IMS external network mode. Through the analysis about the four schemes, the supported business types and the investment of each mode, it recommended the callback method as the company's external network mobile business application solutions, so it can meet the need with both security requirement and business interconnection. The internal network businesses interconnection solution could provide voice, video, conference , phonebook and suchlike multimedia services with APP software. At last, it brought out the business interconnection ways between internal and extern network to realize multimedia communication of the whole network.