The gear has a strong meshing and shearing effect on the gear oil when affected by the load and speed in the operation.To study the effect of shearing on the performance of gear oil,the shearing experiments of the Polymer Alpha Olefins (PAO) were carried out by using FZG gear machine.The effects of shearing on antifoaming,antioxidation and tribological performance of the different viscosity PAO were evaluated.The results demonstrate that,PAO base oils have excellent shear stability,and the shear stability of PAO40 is better than that of PAO10 and PAO4;The shearing action causes the decrease of the antifoaming and antioxidation properties of PAO4,PAO10 and PAO40,but it has little influence on the tribological properties of the PAO base oils.