Abstract: Research on online process monitoring and fault diagnosis (FD) in nuclear power plants (NPPs) helps operator timely know the state of system and do accurate operation in case of accident deterioration. The signed directed graph (SDG) can show the complex relationship between parameters and has advantages of establishing model conveniently, flexible inference and so on. So the SDG was adopted for FD in the paper. Firstly, the combination of qualitative trend analysis (QTA) and threshold was applied to process monitoring. Secondly, the SDG was utilized for FD and revealed the fault propagation path. Finally, the SDG model of the secondary-loop system in NPPs was built to verify the proposed method by simulator in the paper.