The manifolds of airbag inflator were used to be produced by sheet-metal forming. It comprised at least eleven forming stages to achieve its final shape. Uneven wall thickness and poor dimensional accuracy were the common defects because the planar anisotropy of sheet-metal caused earing on the cup rim. In this study, the cup-shape workpiece is obtained from backward-extrusion of billets, followed by two stages of end-forming of tube and three stages of hole-piercing. During the end-forming production, the tool life of the first forming stage was the most severe because it applies close-die forging to increase its rim thickness. Therefore, in addition to the design of prestressed die insert, special attendance is paid to the partition of the die insert to further alleviate the stress level. The optimization of die insert was aided by the finite element analysis of DEFORM software. The results show that by lowering the parting line, the stress level can be minimized. The tool life can therefore be improved.