The invention discloses a double-rack locking mechanism and a large-scale heavy-load motion platform system. The double-rack locking mechanism comprises a retaining frame, locking mechanism pull rods, clasping clamps, a clasping driving cylinder, clasping wedges, clasping clamp pull rods, guide rails, upper wedges, movable teeth, laser range finders, nylon slider devices, a lower lining plate, a movable tooth driving cylinder and an upper wedge driving cylinder. The large-scale heavy-load motion platform system comprises movable beams, uprights, fixed racks and double-rack locking mechanisms, wherein the fixed racks are vertically fixed on the uprights; the double-rack locking mechanisms are mounted at the two ends of the beams; and the beams and the uprights are connected through the double-rack locking mechanisms. According to the motion platform system, the laser range finders are utilized to judge whether the movable teeth reach meshing positions or not, and the movable tooth driving cylinder drives the movable teeth to completely and favorably mesh with the fixed racks; the positioning accuracy of the beam is higher; the upper wedges are inserted into gaps between the movable teeth and the retaining frame, so that reliable locking can be realized; and after loading of test pieces, unlocking can be successfully realized.