At present, the video stream scheme based on Unicast is adopted in VOD systems. It not only increases network load and delay, but also causes user's QoS to be limited by network transmission bandwidth. And networks congestion caused by characteristic of IP networks affects video applications seriously. So a design of network structure of VOD system base on IP Multicast is proposed in the paper. And this system has a predominant scheduler algorithm to Multicast transmission of video stream. At the same time, design and performance of the system's video sever are analyzed. Further, The ways of coding technology and runoff control are proposed, so that QoS will be improved. The main propositions are as follows:1. Propose a design of network structure of VOD system base on IP Multicast. 2. Analyze the design of access sever for system users. including analysis of the most users, analysis of network transmission for users in different regions demanding the same program, and how to access user severs nonblocking for users in different networks, and so on. 3. Propose a way of realizing system QoS, including network optimization, runoff and congestion control of the network, and prior control of different service in the network.