Abstract: Taking the involute spur gear as the research object and considering the thermal effect and time-varying effect, the oil-water two-phase flow model and the thermal elastohydrodynamic lubrication model of the gear system were established to explore the influence of water content in lubricating oil on the viscosity of the oil-water two-phase fluid and the lubrication characteristics of the tooth surface. The results showed that when the water content in lubricating oil was less than 5%, the viscosity of oil-water two-phase fluid increased with the increment of water content. When the water content was 5%, the viscosity of oil-water two-phase fluid reached its peak value, and then decreased with the increment of water content. Appropriate increase of water content can improve the lubrication characteristics of tooth surface, but considering oil film temperature rise and corrosion wear of equipment, the water content should be controlled within 5%.