Taking the high-speed wheel reducer of an electric vehicle as the research object, the systematic efficiency model of the reducer was established considering the gear meshing loss, oil mixing loss, windage loss, bearing loss and other factors. Then, the influence of different operating parameters and design parameters on the various losses and total efficiency of the reducer were investigated. The results show that the system efficiency decreases with the increase of operating parameters of rotating speed, torque, friction coefficient, tooth height and kinematic viscosity. And the rotating speed, torque, and friction coefficient have an obvious influence on the systematic efficiency. But the tooth height multiple and kinematic viscosity have unsubstantial influence on the systematic efficiency. The increase of the pinion teeth number and normal module tends to increase the systematic efficiency first, and then decrease it. The increase of the helix angle can significantly increase the systematic efficiency.