Abstract: The digital pressure control device for the pressurizers in nuclear power plants, which is equipped with the input module of selecting median from four inputs, was used as the research object, and a fault tree including the static part and the dynamic part of the device is established. Next, the Method of Markov was used to analyze the dynamic part, and then the overall fault tree was analyzed according to the overall logic. Finally the reliability of the input module and its effect on the entire device were evaluated from the value of reliability and importance. The evaluation concluded that the input module is relatively highly optimized in terms of reliability.