The gas penetration behavior and the processing parameter determination in gas assisted injection molding(GAIM) was studied.The experiment was performed in a shape tube with a flat surface.And the effects of several important parameters on the remaining wall thickness(RWT)and the length of gas penetration were analyzed.The result revealed that the parameters，such as gas delay time，gas injection pressure and the volume(%) filled melt were the important factors affecting the changes of the length of gas penetration and the RWT in GAIM，and the interaction of these parameters caused variation in the effect of a single parameter.