Human error is still one of the main factors threatening the safe operation of nuclear power plants. In view of the deviation of operators’ actions from operating procedures and the impact of implementation of operating procedures on safety functions, this paper proposes the Operating Procedures Supervision System (OPSS). The OPSS monitors operators’ performance deviating from operating procedures, determines the operation state of a nuclear power plant according to realization of the three key safety functions of the plant (reactivity control, radioactivity containment, and residual heat removal), and evaluates the impact of the implementation of operating procedures on the safety state. Although the OPSS supports both off-line and online applications, this paper presents only an example of off-line evaluation to illustrate the method. On this basis, the paper proposes a risk matrix to comprehensively weigh operator action deviation and the impact of operator actions on plant safety functions, which can more finely perceive the changes of system safety status and of operator performance.