Abstract:Causal diagram analysis is often used in fault diagnosis. Developed from brief network[1], totally based on probability theory, the causal diagram theory with good theoretical basis is a wide range of applications, such as fault diagnosis for there is no limit to network topology. Qualitative analysis is widely applied in fault diagnosis, and specific methods include structure importance coefficient method, the probability of importance and critical degree method. This paper analyzes the cause of fault by structure importance coefficient method, and then takes preventive measures.