Abstract: The demultiplexor plays a key role in set top box（STB）.To customer, it provides two important functions in the demultiplexor-choose the channel and withdraw the data which the customer interest. The characteristics of interface functions of demultiplexor in STB are comprehensive, simple, general, easy to use and of high abstractness. There are some interface functions of demultiplexor in STB, they support every kind of requests of application program and mainly describe demands of application program. According to MPEG and DVB standard, they don’t involve the operation details of demultiplexor, and can run on multiple hardware platforms, providing a stable development environment and nicely supporting the development of application program. Here discussed are their guiding ideology, concept, structure and the services they provide in detail. In addition, the key algorithm that can implement these functions is also given.