To ensure long service life of dies in the cartridge case extrusion forming, the combined extrusion die for cartridge cases was newly designed by finite element simulation based on cold extrusion process theory. Diameters of each layer of the combined die and shrink ranges between neighboring layers were calculated using lame optimal formula, and extrusion forming process for cartridge cases was simulated with DEFORM-3D software. It was proved that the inner wall of combined die can be completely free from tangential tensile stress, and the corresponding structural strength and bearable load was 1.75 times of integral die. So, the fracture risk would be reduced effectively and the cartridge case extrusion forming was achieved with the suitable design of pre-stressed combined die.