The utility model relates to a mechanical forming device for aluminum-plastic packing machines, which comprises a gear box, a main shaft, a cam, an idler wheel, a middle guide plate, a concave mold, a concave mold seat, a cover plate and an upright post, wherein the cam is arranged on the main shaft in the gear box, the cover plate is supported on the gear box by the upright post, the middle guide plate can move upwards and downwards along the upright post, the concave mold and the concave mold seat are fixed on the middle guide plate, the idler wheel is arranged on the bottom surface of the middle guide plate, and the circumference of the idler wheel contacts the circumference of the cam. The utility model also comprises a convex mold and a material shedding plate mechanism which are arranged under the cover plate. The utility model has the characteristics that formed foams have better quality, big forming volume and high forming speeds.