The invention belongs to the technical field of workpiece clamping devices or workpiece positioning devices and particularly discloses a tool for gearbox machining. The tool comprises a rack, a mounting unit, a jacking unit and a pulling unit. The mounting unit comprises a mounting seat, a lantern ring, a vertical column and a turning-over mechanism; the turning-over mechanism comprises a piston cylinder and a piston, a supporting column is arranged on the piston, a turning-over cylinder is hinged to the supporting column, and a closing door is hinged to the turning-over cylinder; and a sliding groove is formed in the side wall of the closing door, and a connecting block is slidably connected in the sliding groove. The jacking unit comprises a rotating roller;and clamping teeth are arranged on the surface of the rotating roller, a clamping groove is formed in the rotating roller, a jacking rod is slidably connected in the clamping groove, and a second spring is connectedbetween the jacking rod and the clamping groove. The pulling unit comprises a first gear and a second gear; a wire wheel is arranged at the rotating center of the first gear, and a winding wheel with a wound pullingrope is arranged at the rotating center of the second gear; and convex teeth are arranged on the side, close to the piston cylinder, of the pulling rope. The tool is used for adjusting the box clearance between the gears, the whole adjusting process is standard, simple and convenient, and the easy operation property is achieved.