The invention discloses an assembling device for an intermediate crossbeam assembly of a heavy load vehicle. The assembling device comprises a soleplate, an L-shaped moveable plate, an L-shaped fixedplate, a tipping gear and a driving device. The intermediate crossbeam assembly of the heavy load vehicle comprises a left connecting plate, a right connecting plate, back-to-back crossbeams, an upperairfoil surface and a lower airfoil surface. The method for assembling the intermediate crossbeam assembly of the heavy load vehicle includes the following steps: assembling the left connecting plate, assembling the back-to-back crossbeams, assembling the right connecting plate, riveting the lower airfoil surface, riveting the upper airfoil surface, and finally checking. The invention has the good effects that the assembling device is simple in structure and convenient to process; the connecting plates can be moved by the L-shaped moveable plate and the automobile assembly crossbeam can be overturned by the tipping gear, thus reducing the labor intensity of the workers; generally, the assembling device can assemble the automobile intermediate crossbeam assembly in about one minute; in theriveting and welding processes, the relative positions of the component parts are controlled through a motor, thus improving the quality of the product.