Failure mode and test method analysis are the premise and foundation of testability modeling and design, and one of the important contents of testability research. Aiming at the problem of low level of testability design of existing naval power system, this paper conducts a test study on medium voltage DC switchboard of a certain naval ship by studying and analysing the potential failure mode sorting method of hierarchical definition, definition of severity category and probability level definition of fault mode method. In this paper, the set of components and parameters that need to be monitored for the type I and II fault modes that cause serious damage to distribution boards is obtained, which lays a foundation for testability design.