Abstract: In the process of depressurization during severe accidents for the second generation nuclear power plant, high temperature fluid flowing through the severe accident depressurization valves may cause deformation of the valve body or even cause the valve stem to fall down because of being heated by high temperature fluid, resulting in depressurization failure. In this paper, the spectrum analysis of the high temperature fluid state that the depressurization valve may experience in the process of pressure relief is carried out, and the high temperature fluid state under different pressure relief capacities is obtained. The influence of the valve channel deformation caused by high temperature on the depressurization effect is studied. Although the second type valve has the possibility of valve channel deformation, it can obtain a longer time window for severe accident mitigation. From the perspective of optimizing the severe accident measures, the discharge capacity range of the second type valve 450-600 t/h is recommended.