The packing stage is the most important stage to plastic parts quality. If the packing curve is not set reasonably, non-uniform distribution of pressure is caused in injection mold cavities, which results in that plastic parts emerge a series of quality problems, such as: warpage, sink marks, and poor dimensional accuracy. Taking into consideration complexity of in the cavities in the packing stage, this paper set packing curve two stages. First stage packing pressure was set at constant so that cavity pressure also kept constant tendency, thus difference of shrinkage rate was reduced, and second stage was set at linear decreasing so that packing pressure hold consistent with cavity pressure that gradually decreased to zero with the melt curing, thereby residual stress was reduced. The packing curve values were optimized, by balancing cavity pressure that is the intermediate variables time after time by the numerical simulation technology. The best packing curve was ultimately got.