Polyalkylene glycol (PAG) gear oil containing phosphorus anti-wear additives is widely used in industrial gear boxes.Due to its strong water absorption,the water content in the oil is often high,which threatens the normal operation of the equipment.In order to study the effect of water content on the properties of PAG gear oil containing triphenyl phosphate,the oil samples with water mass fraction ranging from 0 to 5% were prepared,and the physicochemical properties and tribological performances tests were carried out.The wear scars of steel balls lubricated with different oil samples were characterized and analyzed.The anti-wear mechanism model of interaction among water molecules,triphenyl phosphate,and PAG molecules was established.It is found that the physicochemical properties of oil samples have no obvious change except viscosity when the water mass fraction is 5% or less.However,when the water mass fraction is greater than 2%,the anti-wear performance of the oil sample decreases greatly.This is because excessive water can lead to the gradual expansion of the fluid layer,and the hydrated PAG molecular film will become thinner and thinner,resulting in a significant decline in the overall lubrication performance of PAG gear oil.