The invention discloses a gearbox assembling tool. The gearbox assembling tool comprises a bottom plate, wherein a horizontal gearbox positioning device and an output crank arm fixing device connected with one end of an output shaft of a gearbox are arranged on the bottom plate, the output crank arm fixing device is used for positioning an output crank arm on the output shaft, the horizontal gearbox positioning device is used for positioning the gearbox on the bottom plate, and the distance between a crank arm positioning reference surface of the output crank arm fixing device and a horizontal positioning reference surface of the horizontal gearbox positioning device is designed according to the design requirement, so that the distance from the outer end surface of the output crank arm to a cross point of axes of the output shaft and an input shaft meets the design requirement, that is, the length by which the output crank arm extends out of the gearbox meets the design requirement. With the adoption of the assembling tool, the position of the output crank arm on the output shaft of the gearbox can meet the design requirement, and holes can be drilled more conveniently and accurately; besides, the gearbox assembling tool is simple in structure and reasonable in design, and a lot of defects during mounting of the gearbox by a worker are overcome.