Based on the principle of gear engaging and tribology theory, by simplifying spiral bevel gear to spur gear, and by considering the impact of gear contact ratio and time-variant factors including meshpoint relative sliding velocity, normal load, and friction coefficient, the computing model of friction power loss for spiral bevel gear was established. According to the model, the friction power loss was calculated and analyzed by taking the spiral bevel gear of a helicopter tail reducer as research object. The results show that the computing model is reasonable, simple and reliable, and has a reference value to calculate the friction power loss for spiral bevel gears.