Abstract: Nuclear power plants (NPPs) are very complex grey system, in which faults and signs have not certain corresponding connection, so it’s hard to diagnose the faults. A model based on weighted degree of grey incidence of optimized entropy was proposed according to the problem. To validate the system, some simulation experiments about the typical faults of condenser of NPPs were conducted. The results show that the system’s conclusion is right, and the system’s velocity is fast which can satisfy diagnosis in real time, and with the distinctive features such as good stability, high resolution rate and so on.