Video on demand system (VOD) is a cross technique, which integrate the computer technology, network communications technology, multimedia technology, television technology and data compression technology. Along with rapid development of multimedia and network technique, VOD system already applies widely in many domains. In the DirectShow based VOD system, the role of DirectShow is to decode and play streaming media. In VOD, the kernel technology includes server side document management technology, and client server network multimedia transmission technology, client data playback of multimedia technology, multimedia technology and multimedia data cache decoding technology. It is the COM object that executes specific tasks. This paper research the kernel techniques of DirectShow based VOD system according to COM modules, such as multimedia document management, broadcast, buffer and data decoding.