Abstract: Based on the special application conditions of oil powered heavy-duty four rotor UAV, the lubrication state and meshing efficiency of spiral bevel gears lubricated with grease in the gearbox of UAV were studied. The research showed that lubricating grease was a kind of lubricant with relatively complex properties, and a Reynolds equation suitable for the tooth surface of spiral bevel gears was proposed based on the assumption that the lubricating grease was an elastic-plastic fluid. And the direction of entrainment velocity, surface roughness, and extrusion effects were taken into account when establishing the Reynolds equation. The Reynolds equation of grease lubrication for spiral bevel gears was solved. In order to accurately solve the problem, the discrete mesh in the contact area consists of \text275×275 equally spaced nodes. In order to facilitate convergence, the Gauss-Seidel iterative method was used in the low pressure area, and the Jacobian iterative method was used in the high pressure area. The empirical formula of friction coefficient was replaced by the calculation method considering the shear force and the contact ratio of micro convex body. This was due to the consideration of the particularity of tooth surface meshing of spiral bevel gears, so a mixed-thermal-elastohydrodynamic lubrication friction model was established. The tooth surface lubrication and meshing efficiency under various conditions of UAV were studied. These conditions including constant speed cruise, slow climb, and maneuver avoidance. And the effects of different rotational speeds and lubricant viscosities on efficiency were considered. In order to prove the reliability of the Reynolds equation established in this study, the method of literature comparison was used to prove. The results showed that the Reynolds equation of grease lubrication considering roughness proposed in this study was in good agreement with the results in the literature, and compared to the Reynolds equation (assumes that lubricating grease was a plastic fluid), this model was closer to the experimental results, especially in the high-speed shear stage; The tooth surface of the spiral bevel gear was complex, and the lubrication state on the whole tooth surface of the grease lubricated spiral bevel gear could not be estimated approximately by using the oil center film thickness algorithm. This was due to the fact that the pressure, entrainment speed, relative sliding speed, etc. at each meshing point during the meshing process of spiral bevel gears were different, and the properties of lubricating grease could significantly change with changes in contact conditions. In the study of meshing efficiency, it was found that, the friction coefficient of the tooth surface in a meshing cycle was the largest at the node. At low speed, the gear surface would enter the mixed-lubrication state; The change of meshing efficiency in a meshing cycle was closely related to the meshing position, and the efficiency was the lowest at the meshing in point; On the premise of constant speed, the higher the efficiency of grease viscosity, the greater the loss.