By measuring the tangential load on the surface of different vacuum contact object under the action of external pressure,the effect of gas on the surface friction of rough contact object was studied.The friction pair material used in this experiment is aluminum alloy/plexiglass,45 steel/plexiglass.The experimental results show that the friction force of the two friction pairs shows nonlinear variation with the increase of external pressure,while the static friction coefficient of the two friction pairs is first decreased,then tends to be stable,and then is increased again.The vacuum degree corresponding to the minimum value of static friction coefficient of the 45 steel/plexiglass friction pair is larger than that of the aluminum alloy/plexiglass friction pair.The reasons for the change of static friction coefficient are as follows:with the increase of vacuum degree,the density of gas molecules between the contact clearance of friction pair is decreased,which causes the decrease of the number of gas molecule and solid surface molecule that produce friction when the solid begins to move;with the increase of vacuum degree,the positive pressure is increased,which causes the increase of the deformation degree of the material.