The invention provides an assembling device for a front-drive automobile transmission assembly shafting. The assembling device is good in assembling quality. A bottom plate is detachably installed on an installation plate. A first pair of guide rails, a second pair of guide rails and a third pair of guide rails are detachably installed on the bottom plate. The lower ends of the two guide rails of the first pair of guide rails are close to the middle portion of the upper guide rail of the second pair of guide rails and are inclined leftwards relative to the second pair of guide rails. The second pair of guide rails are inclined rightwards relative to the lower edge of the bottom plate. The inner ends of the third pair of guide rails are spaced with the inner ends of the second pair of guide rails and are parallel to the lower edge of the bottom plate. A left supporting frame and a right supporting frame are detachably installed on the bottom plate and are located on the outer sides of the two guide rails of the first pair of guide rails. A first locking device is connected with the outer end of a first supporting block, a second locking device is connected with the outer end of a second supporting block, and the first locking device and the second locking device are detachably installed on the bottom plate. A first long limiting block, a second long limiting block, a first short limiting block and a second short limiting block are detachably installed on the bottom plate, the first long limiting block and the first short limiting block are arranged at the left end and the right end of the second pair of guide rails, and the second long limiting block and the second short limiting block are arranged at the left end and the right end of the third pair of guide rails.