Abstract: The structure of water injection nozzle was designed by a 3D drawing software to meet the experimental requirements. A visualization investigation was performed for the fluid-assisted injection molding of three-dimensional bending pipe with a 120 °~150 ° chamber by the overflow method using a fluid mechanics software. The diameter of water injection nozzle, the delay time of water injection, water injection pressure and melt temperature varied for this investigation, and the influence of these four parameters on the water penetration behavior inside the pipe was analyzed. The results indicated that the internal penetration length of the pipe increased significantly when the diameter of water injection nozzle was set to 7 mm, which also resulted in a thinner residual wall thickness. Owing to the influence from the structure of water injection nozzle, the water injection delay time was 1s, the water injection pressure was 8 MPa, and the melt temperature was 230 °C. The water penetration length increased by 400 % but the residual wall thickness decreased by 20 %. The maximum penetration length of 298 mm was achieved at the water injection delay time of 1 s, the water injection pressure of 10 MPa and the melt temperature of 230 °C. There was a more great influence from the water injection nozzle for the experimental groups with melt temperatures of 210 ℃ and 230 ℃ at the water injection delay time of 1 s, the water injection pressure of 8 MPa and the melt temperature of 250 °C. The hollow rate of the pipes could be improved effectively and their wall thickness could become thinner by shortening the water injection delay time and improving the water injection pressure and melt temperature. However, the influence of water injection nozzle on water-assisted injection molding could be ignored, because any small changes in its structure could greatly change the internal cavity of the molded parts and effectively improved the efficiency of water penetration behavior.