Abstract: According to the design rules of computerized operating procedure system (COPS) for high-temperature gas-cooled reactor nuclear power plant (HTR NPP), the overall scheme of COPS of HTR NPP was proposed. It was pointed out that an important function of COPS was to provide a choice of right procedure. Abnormal event operating procedures for HTR NPP are event-based, so the key to select a protocol entry point is the recognition of anomalous events. To solve the problem, an abnormal event recognition expert system was built for abnormal event operating procedure entrance monitoring of HTR NPP, using the method of fuzzy Petri net, which has excellent graphic description ability and rigorous mathematical theory. Compared with the traditional methods, the method in this paper has great advantages in strengthening the structural features and knowledge expression ability, improves the reasoning efficiency and is convenient for computer programming. The simulation of a typical case of abnormal event recognition verifies the feasibility and effectiveness of the designed system, laying the foundation for the development of COPS of HTR NPP.