Instant messaging, a new method of communication, has served as a method of communication in many businesses. This paper presents a framework for the communication server cluster used in MSRP instant messaging business. It offers good support for the offline message business in SIP system. Considering MSRP’s working under session mode, the paper presents an overall design method, and researches and analyzes the signaling process of offline message storage and transmission. Then, the server cluster implementation schedule and the communication framework inside cluster is also presented. The server system is capable of handling the MSRP offline messaging business in cooperation with the SIP softphone.