The invention discloses a thread pre-tightening and locking device for a well drilling pump valve cover. A pressing cap, wedge blocks and a gland are sequentially mounted from top to bottom in a step hole of a hydraulic cylinder body; the pressing cap is in threaded connection with the hydraulic cylinder body; a plurality of wedge blocks are arranged at the lower part of the pressing cap; all wedge blocks are concentrically arranged to form an approximate ring; all wedge blocks are together provided with a spring along the outer circumferential direction; the lower end of a bolt is arranged in a space formed by the inner surfaces of all wedge blocks; a locknut is mounted on a screw rod at the upper end of the bolt after the screw rod passes through the bottom end face of the pressing cap; all wedge blocks sit on the upper surface of the same gland; a blind hole which sleeves the lower end of the bolt is formed in the center of the upper surface of the gland; and the gland is in hermetic contact with the step hole of the hydraulic cylinder body by virtue of a valve cover sealing ring. The device disclosed by the invention is reliable in pre-tightening and locking and especially applicable to a valve cover of a high-pressure well drilling pump.