The invention relates to the technical field of car part machining, in particular to a device for car gear shaft machining. The device comprises a U-shaped pipe filled with seepage-proofing coating, one end of the U-shaped pipe is connected with a pressure device, the other end of the U-shaped pipe is connected with a positioning plate in a sealed manner, a through hole is formed in the positioning plate, and a vertically-arranged guide pipe is connected into the through hole in a sealed manner; and a positioning hole is further formed in the positioning plate vertically, the lower end of the positioning hole is fixedly connected with a sealing ring, a base plate connected with the positioning hole in a sliding manner is further arranged in the positioning hole, the base plate is in clearance fit with the hole wall of the positioning hole, and a height controller is connected to the base plate. By means of the device, the coating accuracy of the seepage-proofing coating can be improved, and the product quality of the gear shaft can be guaranteed.