The mean sliding friction factor was derived in the mixed elasto-hydrodynamic lubrication state, which contains the complete elasto-hydrodynamic and boundary lubrication synchronously. Based on the geometry and load contact characteristics of spiral bevel gears, the power loss from sliding friction of each point on the long axis of the instantaneous contact ellipse was calculated. Meanwhile, the function of sliding friction power loss in a mesh cycle was established. The meshing efficiency was obtained by integrating the above function. In addition, the meshing efficiency was increased through optimizing the contact parameters. The validity and superiority of the algorithm was demonstrated with analyzing an example.