The main research work and its technical characteristics of this article are: Apply Java language, MySQL database to complete the design and development of instant messaging software on the basis of cross-platform Qt development framework and Android operating system, reflecting the cross-platform design ideas; the introduction of a multi-threaded server based on decision tree and decision-making mechanism, after the original user message encrypted MD5 algorithm, information is placed in UDP packets for network transmission, so that the system has excellent time response and security feature. In order to optimize the man-machine interface, with the QT development platform, the client adopt the popular MVC design pattern and decorative design pattern.