Abstract: Expert system has successfully resolved fault diagnosis of complicated system. Its core is knowledge base and inference machine. In this paper, fault tree was used to express the logical relations of faults in the di esel control systems on a certain ship. Then, it was translated into binary fault tree, which could design the knowledge base and inference machine of expert system, after being combined with the expert system. This me thod was used to diagnose the fault of diesel control system on a certain ship. The results show that the storage structure of expert system is simple and easy to maintain, and the logical inference process of inferenc e machine is concise and easy to carry out.