This paper firstly establishes a mathematical model of ship power system, and then analyzes the characteristics and common faults of ship power system. D-S evidence theory method is used on research of common faults of the ship power system, to enhance the pertinence of fault diagnosis. By using multi-source information fusion diagnosis, the need for quantities of electrical data is reduced, and, it can effectively reduce the impact of protection or switch malfunction on the fault diagnosis of ship power system and thus improve the accuracy of diagnosis.