This paper investigates process optimization for the specific rib with section s-3 during the gas-assisted injection molding firstly, which is to get relationship between process parameters and quality evaluations,based on the Taguchi experimental design. Then, in the light of the above process conditions, a comparative study of four different cross-section ribs are done, to investigate trend of the impact of the single process parameter on forming quality, by changing one of the parameters while the others remain unchanged.