Abstract: In order to identify main human error modes of operator in digital main control room of nuclear power plant (NPP), the main cognitive functions of operator were identified based on field observation, operator interview, and cognitive activity analysis. The classification framework of human error was established on the basis of identified cognitive functions of operator. The main human error modes of operator were identified by the full-size simulator experiment, and the causes leading to human errors were analyzed. It provides theoretical support for improving operators’ reliability, reducing and preventing human errors.