The average sliding friction coefficient was derived in the mixed elastohydrodynamic lubrication state, with complete elastohydrodynamic and boundary lubrication. Based on the geometry and load contact characteristics of spiral bevel gears, the sliding friction power loss of each point on the long axis of the instantaneous contact ellipse was calculated. Meanwhile, the function of sliding friction power loss in a meshing period was established. The meshing efficiency of spiral bevel gears was calculated by integrating the function under the condition of mixed elastohydrodynamic lubrication. This algorithm was demonstrated to be correct and precise compared with Gleason corporation’s method of calculating the meshing efficiency of bevel gears.