The invention provides an automatic pre-tightening device for ball screws. The automatic pre-tightening device is composed of a ball screw, a clearance-eliminating ball nut, a lock nut, a torque applying nut, an energy storage spring, a left spring pile, a right spring pile, a pre-tightening nut, an end bearing, an outer sleeve, a flat key, a transmission ball nut and a ball, wherein the clearance-eliminating ball nut and the transmission ball nut sleeving the ball screw are closely mounted in the outer sleeve; and the end bearing, the pre-tightening nut, the energy storage spring, the torque applying nut and the lock nut are mounted on a fine pitch thread arranged on the outer surface of the clearance-eliminating ball nut. The torque applying nut applies pre-tightening torque to the energy storage spring, the energy storage spring stores energy and transfers the pre-tightening torque to the pre-tightening nut, and the pre-tightening nut applies axial thrust to the outer sleeve through the end bearing under the torque action of the energy storage spring. When a ball screw assembly gets clearance, the transmission ball nut generates relative axial displacement relatively to the clearance-eliminating ball nut, and the transmission clearance between the transmission ball nut and the ball screw is eliminated.