The invention discloses a prestretching mounting structure for a screw rod. One end of the screw rod is fixed, and the other end of the screw rod is free; bearings are separately arranged at the fixed end and the free end; the bearing at the fixed end is arranged in a bearing block fixed with a machine tool main body structure; a bearing bush is arranged outside the bearing at the free end of the screw rod; a motor base connecting flange is arranged outside the bearing bush and is fixedly connected with a motor base; a through hole is formed in the motor base connecting flange; a longitudinal sliding chute is formed in a position, corresponding to the through hole, of the bearing bush; a thread section is arranged at the outer side of the bearing bush; a thread section part is located at the lower side of the motor base connecting flange; part of the thread section part extends from the outer side of the motor base connecting flange; and a bearing bush adjusting nut is screwed in the thread section. The bearing bush adjusting nut can screw in the thread section of the bearing bush by a certain distance, so that the prestretching amount of the screw rod can be controlled, further the control over the prestretching amount of the screw rod is conveniently realized, and the simplicity in operation is realized.