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With the rapid development of the mobile Internet, mobile instant messaging (IM) systems are playing increasingly important roles in our life. However, it still faces with the issues of security and connectivity. XMPP (the Extensible Messaging and Presence Protocol), which is a cross-platform and open protocol, was adopted in this paper to resolve the connectivity problem of the IM system. And a hybrid encryption algorithm to secure the system was also designed, using the AES, SHA-1, and RSA algorithms to implement the hybrid encryption policy; the realization of the security algorithm was based on the Bouncy Castle encryption library. The whole system is composed of the Openfire server based on XMPP, and the mobile clients implemented in the Java ME language. The mobile client comprises four modules of the interface design, XML parsing, XML packaging, and event response. The main functions implemented in this mobile IM system include user login, roster, status change and display, sending and receiving of instant and encrypted messages, buddy management, chat record management and so on. This system is not only suitable for ordinary users, but also fit for enterprise applications.