The invention discloses a mounting tool of a ship shafting case butt coupling. The mounting tool comprises a frame body and a derrick assembly, wherein the bottom end of the frame body is vertically and fixedly connected to a shaft body through a first permanent magnetic chuck, a guide rail is arranged on the frame body in the X axis direction, the derrick assembly comprises a base capable of moving along the guide rail back and forth and a derrick fixedly connected to the bottom of the base, and a Y-direction hand wheel for adjusting the derrick to move in the Y-axis direction and a Z-direction hand wheel for adjusting the derrick to move in the Z-axis direction are arranged on the base; and a second permanent magnetic chuck is arranged at the bottom end of the derrick to adsorb the coupling, and a camera device used for shooting the real-time position of the coupling is arranged on the derrick. The invention further provides a mounting method of the ship shafting case butt coupling.According to the mounting tool of the ship shafting case butt coupling, the hoisting safety of the coupling is high, besides, centering during mounting of the coupling is simple and accurate, the labor intensity is reduced, the working efficiency is improved, and the production cost is reduced.