Abstract: The main steam isolation valve is the most important isolation equipment of the steam pipeline between the nuclear island and the conventional island in nuclear power plants. This paper mainly introduced the function and control theory, and the failure mode analysis. Combined with the historical fault statistics, the weak point of the system-limit switch is obtained, and the failure mechanism and root cause analysis of limit switch failure are emphatically expounded. At last, the paper puts forward the improvement measures from five aspects: human, machine, material, method and environment, which has certain reference significance for the maintenance and improvement of the main steam isolation control system of other nuclear power plants in service in China.