The invention relates to a device for automatically eliminating clearances of a screw drive pair, which comprises a leading screw, a main nut, a bolt, a machine tool sliding plate, an adjusting bolt, a spring, a guide post, a nut with an inclined plane, a bolt, a locking nut, a wedge block and a ball. The device is characterized in that the main nut and the nut with the inclined plane are connected with the leading screw respectively, the main nut is connected with the machine tool sliding plate through the bolt, and the flange of the nut with the inclined plane is provided with a through hole so that the nut with the inclined plane can move axially along the guide post. The wedge block with a self-locking function is supported between the main nut and the nut with the inclined plane; andone end of the spring is pressed on the wedge block with the self-locking function, while the other end is tightly pressed by the adjusting bolt. The incline plane of the nut with the inclined plane is provided with a guide groove. The device automatically eliminates axial clearances, has no reverse idle stroke, has adjustable pretightening force, good axial rigidity, smooth transmission, low cost, compact structure and high space utilization rate, is applied to feed drive systems of a metal cutting machine tool and a numerical control machine tool, and can greatly improve the processing accuracy.