The invention discloses a device and a method for testing the precision retaining ability of a ball screw assembly. A head frame of the device is fixedly arranged at one end of a bed body; an output shaft of an alternating-current motor is connected with an output shaft of a speed reducer through a synchronous belt transmission mechanism; the other end of a tested ball screw is arranged in a mounting hole in a tail frame and can rotate; a fixing plate is fixedly connected with a second group of slide blocks; the second group of slide blocks has two slide blocks; the two slide blocks are respectively locked on two guide rails; the two guide rails are fixedly arranged on the bed body and are parallel to the axis of the tested ball screw; a nut penetrates through the tested ball screw and is fixedly connected with a nut base; the nut base is fixedly arranged on a slide plate; the slide plate is fixedly connected with a first group of slide blocks; and the nut and the slide plate drive the first group of slide blocks to move on the guide rails under the driving of the rotation of the tested ball screw. The device can test and detect the precision retaining ability of the ball screw assembly, is convenient to operate and is reliable in a detection result.