According to the heat transfer theory and the thermal network method, and taking the spiral bevel gear transmission system of a helicopter tail reducer as research object, the calculation model of the temperature field Simulink simulation was established. Based on the model, the steady-state temperature field of the transmission system was calculated in the normal lubrication. Taking it as the initial condition, the transient temperature field of the transmission system was calculated and analyzed in the condition of loss of lubrication. It provides the theoretical foundation of predicting the survivability of the gear transmission system in the condition of loss of lubrication.