Abstract: To improve the security and reliability of marine nuclear power plant (MNPP), and insure fault diagnosis effectually and fast, according to fault characteristic of MNPP, a fault diagnosis method based on expert system (ES) and knowledge representation (KR) was introduced. It is combined with ES, KR and reasoning machine(RM) as an integration. The results indicate that fault diagnosis method based on ES and KR can diagnose complexity fault of MNPP and availability enhance the security and reliability of MNPP.