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Composite pipe fittings with an outer layer of 20% long glass fiber-reinforced polypropylene (LGFR-PP) and an inner layer of polypropylene (PP) were prepared via water-powered projectile-assisted co-injection molding short-shot (W-PACIM-S), water-powered projectile-assisted co-injection molding overflow (W-PACIM-O), gas-powered projectile-assisted co-injection molding short-shot (G-PACIM-S), and gas-powered projectile-assisted co-injection molding overflow (G-PACIM-O)techniques. The effects of different injection molding processes on the wall thickness, inner surface roughness, glass fiber orientation, and pressure resistance of pipe fittings were studied to evaluate the quality of the pipe fittings formed by each process. Compared with the short-shot method, the overflow method results in pipes with thinner walls in each layer, a more uniform distribution, smoother inner wall surfaces, and better orientation of glass fibers along the axial direction in the near boundary layer, resulting in better pressure resistance. Under the same injection method, the difference in fluid medium did not significantly change the trend of wall thickness variation in each layer. However, compared with gas, high-pressure water improves the uniformity of the pipe wall thickness and inner wall quality. In addition, the introduction of the warhead is more conducive to improving the degree of glass fiber orientation of the pipe fittings, and the thickness of the residual wall thickness of the pipe fittings has a great influence on the pressure resistance of the pipe fittings. Full article