With the rapid development of electronic technology, on-line fault diagnosis of electronic equipment has become a hot research field. After some basic concepts and principles of fault diagnosis are introduced, this paper focuses on the diagnosis method based on knowledge technology, expounds in detail the diagnosis principle and research achievement such as the fault tree, pattern recognition, expert system, neural network, and the rough set. Finally, the new development and the problems of the fault diagnosis technique are summarized, looking to the future development of fault diagnosis technique which is based on Knowledge technology.