Water-assisted injection molding of short glass fiber reinforced polypropylene composite effectively combines the advantages of fiber reinforced polymer composite and water-assisted injection molding technology, and has become the focus of attention. Residual wall thickness is an important index to measure the products of water assisted injection molding of overflow method, and it is used to describe the water penetration behavior. In this paper, the sensitivity of water penetration behavior on the main process parameters of the overflow waterassisted injection molding was studied. It is found that the mass fraction of short glass fiber is most sensitive to water penetration behavior. For example, when the mass fraction of short glass fiber is 30% , the residual wall thickness varies by 0.87 mm along the axial direction. The second is the water injection delay time and melt temperature. For example, when the water injection delay time is 0 s, the axial direction variation range of the residual wall thickness is 0.76 mm, and when the melt temperature is 270 ℃, the axial direction variation range of the residual wall thickness is 0.75 mm. The last is the water pressure. For example, when the water pressure is 4 MPa, the residual wall thickness varies by 0.39 mm along the axial direction.