Abstract: In order to solve problems of wasting network bandwidth and server overload existed in traditional real-time streaming data transmission mode of unicast and IP multicast,the paper put forward a method of building multicast tree of application layer of overlay network based on immune algorithm. The method uses immune algorithm to divide multicast island and select multicast service nodes with the minimum system cost by taking round trip time of links and degree of nodes as constraints,and realizes the global optimal selection of multicast service nodes. The simulation results showed the method is effective,and has faster convergence speed and better search ability than traditional genetic algorithm.