The method of molding in barrel is a new technology of extruding pure UHMW-PE pipes continuously on single screw extruder. Its significant advantage is that the pure UHMW-PE tube is extruded continuously and efficiently without any additives. In order to get UHMW-PE pipe with smooth surface, high dimensional accuracy and excellent performance, the tube should be molded precisely by the gas-assisted device. This paper will analyze the principle of UHMW-PE pipe gas-assisted precision molding and simulate the process. The analytical results such as temperature, time and internal stress distribution provide a theoretical basis for the design of the gas-assisted precision molding device.