Abstract: In order to investigate the influence of oil-water emulsion on the thermoelastohydrodynamic lubrication characteristics of gears. Based on the physical properties of oil-water emulsions, a steady-state thermoelastic flow lubrication model for involute gears with line contact considering oil-water two-phase fluid as well as thermal effect is established, and the lubrication model is solved by the multiple mesh method and multiple mesh integral method, and the relationships of oil film pressure and film thickness with water content, rotational speed and torque are analyzed. The numerical calculation results show that, under the oil-in-water flow type, with the increase of water content, the emulsion viscosity increases, the film thickness increases, and compared with the pure oil lubrication, the oil-water emulsion lubrication has better lubrication performance; with the increase of gear speed, the film thickness increases, and the lubrication performance is enhanced; with the increase of torque, the pressure increases, the film thickness decreases, and the lubrication performance is weakened. Finally, with the film thickness ratio as the criterion, based on the calculation of water-in-oil lubrication, the lubricant viscosity in the oil-water emulsion required for the formation of an effective lubricant film in gears under different working conditions is predicted, which provides a theoretical basis for the design of lubrication performance and viscosity prediction of oil-water two-phase gears.