Abstract: This paper first introduces the establishment of a transformer fault diagnosis model, and then describes the overall design, main functions and business applications of a transformer fault diagnosis expert system. The advanced fault diagnosis techniques exploited in this paper follow practical use habits, and has no missing or wrong judgment during diagnosis process. It can provide rational and realistic maintenance decision based on property, severity and status of the fault. This system provides a good software platform for accurate fault diagnosis and maintenance of transformer equipment, and an effective method to reduce power generation and transmission cost.