Enterprise instant messaging(EIM) technology is a widespread research hotspot recently. Through the analysis of EIM of C/S architecture, B/S architecture, RIA architecture application technology, this paper thinks EIM based on RIA architecture has many advantages. It gives the principle of Flex technology development EIM system based on RIA, and describes multi-user interaction relationship and core functions of message interactive, audio and video interaction, user information management, global state variables management. It realizes EIM system based on RIA. Project practice shows that EIM system based on RIA has certain practical value and commercial prospects.