The paper studied and constructed an expert system for vibration fault diagnosis in large steam turbine generator set, and briefly described the structure and function of various components of the expert system, detailed described the knowledge database and inference machine. In this expert system, the knowledge database built on production rule, and the inference machine based on confidence. Finally, the expert system is verified through the experiments of collision between movable and stationary parts and gas exciting. The experiment results prove that the expert system has good performances.