The quality of the green and the shaping efficiency are affected by the mold structure during the course of powder injection molding (PIM). Based on the numerical simulation, the mathematic model for the optimal design of the gating and runner system in PIM mould is constructed. The method is testified by the Moldflow software and the experiment. From the result, it is suggested that the clamp force and the shape cycle are clearly decreased, the green is densified and the samples’ density error of the green is decreased after the optimal design of the mold structure. Furthermore, the number of trial mold is reduced and the shape efficiency is improved.