In the extrusion of polyvinyl chloride (PVC-U) round tubes, the melt is prone to adverse effects such as demold expansion and melt rupture after leaving the mold. In order to improve the quality of PVC-U round tubes, gas-assisted extrusion molding technology was introduced; by using the numerical simulation method, the two-dimensional "gas-liquid-gas" coupling flow model was established by Polyflow software, and the changes of the overall dimensions, melt velocity field, shear rate and first normal stress difference of PVC-U round pipes at four intake angles of 15°, 30°, 45° and 60° were systematically explored. The results show that the larger the inlet angle, the smaller the shrinkage rate of wall thickness, the inner and outer diameter dimensions and the wall thickness at the melt outlet are not significant, the angle is too large to weaken the gas dragging effect, reduce the axial velocity, and the maximum value of the first normal stress difference of the melt increases, which is not conducive to production efficiency and quality. The extreme value of the radial velocity of the melt is too large (0.58 mm/s) due to the small inlet angle, which cannot fully inhibit the extrusion expansion of the melt. When the inlet angle is 15°, the peak shear rate is the maximum, which is easy to induce flow instability, and the inlet angle of 30° and above can effectively inhibit excessive shear (15.2% reduction compared with 15°) and improve the surface quality. Therefore, it is concluded that the inlet angle of 15°~30° can take into account the dimensional accuracy and flow stability.