The invention relates to a forming method of a cylinder workpiece with a transverse inner rib. According to the forming method, the procedures of blanking, upsetting and backward extrusion are adopted, and the forming method is characterized in that the procedures of grooving and overturning, overturning a blank inner rib die and the like are adopted; a male die of the overturning blank inner rib die comprises a male die correcting ring, a male die pressing plate, a connecting rod mechanism, a spring, a pressing rod, a limiting pipe and a pressing ring; and a female die comprises an inner overturning female die body, an inner overturning female die body supporting ring and a lower die plate. The problems that a cylindrical part with a large body and a small opening is extruded, and demolding is difficult are solved, a female die opening is designed to be a large-arc-shaped fillet, the shearing stress generated when a thick-wall annular rib at the upper end of a backward extrusion cylinder workpiece is overturned is reduced, material breakage during overturning is prevented, and the quality of the outer surface of an extrusion cylinder is improved; and materials are saved, efficiency is high, and the forming method is suitable for batch production.