Transformer fault is the long-term accumulation complex result of transformer itself and its application environment, so the symptoms of transformer fault is also diversified and the connection between fault symptoms and fault mechanism is complex. Lot of work staff members at the scene lacked of knowledge of fault diagnosis, and they can not frequently asked experts in the field, so carrying out the research and applications of transformer fault diagnosis expert system, to ensure the transformer long-term, reliable operation, which has an important meaning and application prospects. This paper analyzes the current research status of transformer fault diagnosis expert system, based on specific application, to study the structure and key technologies of power transformer further. The most important issues of fault diagnosis expert system are knowledge representation and reasoning mechanism. According to the characteristics that human experts diagnose the fault of transformer, on the basis of learning the process that human experts diagnose the fault of transformer, the paper analyzes and discusses in detail the system structure, knowledge representation and reasoning mechanism to build fault diagnosis expert system of transformer.