Abstract: The diagnosis of the operational state of a nuclear power plant (NPP) plays an important role for the safety and reliability of NPP operation. In this paper, the qualitative method for fault diagnosis based on signed directed graph (SDG) was applied in a complex NPP system because the mathematical model of NPP is difficult to be built. The reactor coolant system (RCS) was taken as the diagnostic object, and the approach of building SDG model was presented and the SDG model of the RCS was built. Based on the SDG model, a fault diagnosis system of RCS was developed, and the steam generator tube rupture (SGTR) and rod ejection accidents were taken as example to analyze the process of diagnosis inference. The simulation results show that the method based on SDG can effectively diagnose the fault in RCS, and it can also provide good explanation for the fault propagation paths. Therefore, this method can help operators to make correct decisions.