Taking the reducer casing of unmanned helicopter transmission system as the research object, the thermal-fluid-solid coupling calculation model is established. The flow field and temperature field in the reducer cabin is simulated based on Fluent. The effects of the airflow velocity and direction of the front air-inlet, the position of the side vent, and the number and position of the heat sink on the heat dissipation Performance of the casing are analysed. The results show that, with the increase of airflow velocity, the temperature distribution pattern of the casing wall is consistent, but the overall heat dissipation performance is enhanced; the airflow direction of the air inlet can directly change the flow field in the cabin, which significantly affects the temperature distribution of the outer wall of the casing; the position of the side vent can change the airflow distribution in the cabin, which indirectly affects the temperature field on the wall of the casing; and the the additional cooling fins on the casing surface can significantly reduce the temperature of the magazine wall and improve the thermal performance of the casing. The simulation results provide a basis for determining the percentage of heat dissipation in the casing, selecting the location of the heat sink, designing the of air-inlet, and arranging the side vent during the development stage of the gearbox.