The end cap of fuel tank is used to close the end of automobile fuel tank cylinder to isolate the internal and external media. For the difficulty in smooth shrinkage of negative angle at the mouth during the forming process of aluminum alloy fuel tank end cap, according to the forming characteristics of the fuel tank end cap, it was proposed to adopt a special wedge mechanism to realize the one-time forming of lateral circumference, and by increasing the suction ribs to plan the flow direction of material at the mouth of end cap when closing, the rigidity of the part was improved while ensuring that it matched the shape and contour of cylinder clarity to meet the welding needs. Practice has proved that through the above process improvement and structure design of the closing die not only improve the dimensional accuracy and appearance quality of end cap products, but also reduce the difficulty of assembly and improve the production efficiency.