Abstract: Water-assisted co-injection molding (WACIM) technology is an novel injection molding process that combines co-injection molding technology and water-assisted injection molding technology. Its special process makes the glass fiber reinforced composite used in the process have special characteristics of fiber orientation and enhancement. WACIM pipes with polypropylene (PP) as inner material and short glass fiber reinforced polypropylene (GF/PP) as outer material were prepared to investigate the effects of glass fiber mass fraction on the thicknesses of pipe inner and outer layers, glass fiber orientation and tensile strength. It is found that when the mass fraction of glass fiber is less than 30wt%, the variations of pipe inner and outer layer thicknesses are not obvious. When the mass fraction of glass fiber increases to 40wt%, both the inner and outer layer thicknesses of pipe increase. According to the distribution characteristics of glass fiber orientation, the outer layer of WACIM pipes can be divided into three layers: Near interface layer, intermediate layer and near mold wall layer. The orientation degree of glass fiber along the flow direction decreases from inside to outside. The tensile properties of pipes increased first and then decreased with the increase of glass fiber mass fraction. The highest tensile strength of pipes can be obtained when the glass fiber mass fraction is 30wt%. It is found by comparison that the influences of glass fiber mass fraction on thickness, glass fiber orientation and tensile strength of WACIM and water-assisted injection molding (WAIM) pipes are different, and the influence mechanism are different.