The pressurizerpilot operated safety relief valves (abbr. POSRV) protect the reactor coolant system against overpressure situation under accident conditions. It can be used us safety valves, relief valves or isolation valves. It’s one of the most important valves in PWR. The POSRV of a nuclear power unit adopts the thermal pilot structure design which is the first application of the third generation nuclear power unit. The POSRV composed of main valve and metallic pilot detector with special sealing design which can withstand the high temperature and high pressure medium in the primary circuit. During the first cycle operation of the nuclear unit, the opening of the isolation valve of the POSRV decreases, the discharge line temperature of metallic pilot detector continuous rise and the leakage rate of the primary coolant circuit increases abnormally, which bring great retreat risk to the unit. Combined with the structural principle of the POSRV and the change trend of key parameters, the analysis shows that the reason for the decrease of the valve opening is the leakage of the three-way sealing surface of the metallic pilot detector, which leads to the introduction of pressurized medium into the valve head of the main valve, then reduces the valve opening and affects the leakage rate of the primary coolant circuit. By formulating mitigation strategies such as energizing the solenoid valve to change the pilot valve state, draining the leakage medium in the primary coolant circuit and the unit operation decision-making scheme, the valve availability and unit safety are ensured, which provides a reference for similar equipment problems.