Residual wall thickness is an important measure of the quality of water-assisted injection molding (WAIM) parts. Based on the traditional injection molding model, Reynolds stress was introduced into the WAIM model for the turbulent characteristics of high pressure water. In addition, residual wall thickness was computed by a computational fluid dynamics (CFD) method, and the results matched well with experimental data. The results show that the hollowed core ratio changes from small to large in the first half of water penetration, while in the latter half it is just the opposite. Difference of the residual wall thickness is larger at the large-diameter position of a part than other position. Residual wall thickness is mainly affected by water injection pressure, delay time, melt temperature, mold temperature and amount of short shot, in which the amount of short shot is the most obvious.