Thin-walled slender tubes are produced by injection molding, where there might be defects such as short shot, excessive warpage and uneven temperature distribution. In this paper, injection process analysis was carried out. The 3D modeling technology of Pro/E established a solid model of thin-walled slender tube. MPI (Moldflow Plastics Insight) was used for executing injection molding flow simulation. Gate location and gate number was optimized. Filling system and cooling system of the combined cavity was optimized by analysis of flow, cool and warp. The results can be used for supplying reliable data of mold designing and manufacturing, reducing the engineering mold trial time, decrease the manufacturing cost. The configuration and dimensions of the injection gate can be determined by the computer program before the molds are actually manufactured. The potential defects were forecasted. Therefore, the designers can quickly optimize the mold design.