Abstract: Due to the severe conditions in severe accidents of the nuclear power plants, the depressurization valves may experience a high temperature transient that the valves cannot withstand and may fail during the depressurization process. In this paper, the typical severe accident sequences that have a certain envelope and contain various typical severe accident phenomena are selected from the accident sequences that may lead to high-pressure melt ejection accident. According to the accident sequence, the high-temperature transient calculation was carried out considering the valves opening time range in severe accident management, and the water level of the pressurizer at the time of valve opening was analyzed for important influencing factors. Finally, the working conditions of the 100 MW nuclear power plant severe accident depressurization valves with typical and certain envelope characteristics are determined, and the maximum possible temperature and the variation temperature curve of fluid passing through the valves before and after valve opening are provided. It provides an important basis for equipment identification and function application of depressurization valves in severe accidents.