The principal factors affecting the warpage of long-armed product during plastic injection process were discussed. Using injection process numerical simulation and design expertise, this paper studied the approach to decrease warpage degree with multi-gates injection. Taken one straight long-armed part as sample, the potential warpage caused by uneven cooling and non-uniform shrinkage with two or three gates injection was studied and predicted respectively with numerical simulation method. The result shows that the number and position of gates has great influence to the product warpage. It is an effective way using simulation method as a virtual tool for conceptual mold design thus to find out design defects, improve mold design quality and optimize injection process.