Abstract: Before overhauling, the Tianwan Nuclear Power Plant has tested the pre-isolation valve of the air relief valve, with numerous tests incorporating timeout restrictions. It is determined during an on-site inspection that there is condensate present within. These phenomena could prevent the internal decay heat from being released in time and potentially start a nuclear fuel explosion if it happens when the nuclear power plant is experiencing an emergency and needs to release pressure. The air relief valve's internal flow field is more complicated and subject to a high temperature and high-pressure working environment, making the experimental method difficult to use for this research. Instead, numerical simulation has gradually grown to be a crucial tool for identifying the factors that influence the opening of the air relief valve, but the majority of the current numerical simulations for the air relief valve do not take the influence of condensate evaporation into account. This study investigates whether there is a phase change in the upper cavity of the air relief valve front isolation valve and the impact of high temperature and high-pressure saturated water film quality on the opening time of the valve disc. To achieve this, ANSYS Fluent is employed to numerically simulate the pressure relief process of the air relief valve front isolation valve used in Tianwan Nuclear Power Station. It has been discovered that when there is saturated water in the top cavity of the valve disc, the valve disc's opening time will be delayed when the upper cavity's pressure is compensated for and the saturated water evaporates. The valve disc's opening time grows monotonically as the upper cavity water film's mass increases, and a mathematical link between the valve disc's opening time and the upper cavity water film's quality is suggested. In order to avoid condensate creation and prevent condensate evaporation, it is advised that nuclear power plants incorporate a heating and insulating function into the valve when utilizing and optimizing it in the future.