Residual wall thickness (RWT) is an important indicator that characterizes the quality of water-projectile-assisted injection molding (WPAIM) pipe fittings, which is directly affected by the penetration behavior of warhead. In order to study the penetration behavior of projectile and corresponding RWT distribution law in WPAIM process, the WPAIM process was numerically simulated based on computational fluid dynamics (CFD) method, and the simulation results are in good agreement with the experimental results. The research results indicate that the RWT of WPAIM pipes is relatively uniform in the straight section, while in the curved section, the RWT on the convex side is greater than that on the concave side, and the difference in RWT between the two is smaller than that of water assisted injection molding (WAIM) pipes. By comparing and analyzing the penetration behavior of warheads in WPAIM with that of water in WAIM, it is found that the pressure distribution near the penetration front is more uniform in WPAIM compared to WAIM, and the velocity difference between the penetration sides is also smaller. The projectile can better penetrate along the center of cavity, improving the uniformity of pipe wall thickness; the penetration process of both is continuously accelerating, whether in the straight or curved section. WAIM penetration speed is faster, while the WPAIM acceleration process is more stable.