Abstract: During the operation of the main transformer rooms, if the heat of the main transformer is not released timely, the environment temperature would exceed the operation temperature, which affects the equipment performance. In order to ensure the safe and stable operation of the main transformer, based on the layout characteristics of ground type main transformer rooms of the hydropower stations, we choose the calculation temperature of three summer ventilations and three main transformers with different capacities to calculate the natural heat release, and obtain the size of natural ventilation hole under various operating conditions. It can provide reference for the ventilation scheme design of the ground type main transformer rooms of hydropower stations.