Abstract: According to the production rules base stored in the nuclear power system fault diagnosis expert system, a neural network (NN) module of expert system was built. The NN inference mechanism including the ability of parallel inference and the knowledge acquisition mechanism was set up. On the basic of the built NN, an inference explaining mechanism was built to it. This system syncretizes advantages of both traditional expert system and NN that it can do case inference and diagnosis, and explain efficiently and accurately as well as effectively acquire knowledge from real cases.