Taking a five-cavity microtube as example, a 3D extrusion flow model of multilumen catheter was established and numerically solved based on finite element method. By analyzing the melt flow field and stress distribution inside and outside the die, the influence mechanism of multi-lumen microtube extrusion molding was studied under traditional and gas-assisted wall slip conditions. The research results indicate that, in the traditional extrusion process, the die swell ratio of the five-cavity microtubule and the ellipticity of the main and secondary cavities all increase with the increase of melt inlet flow rate and relaxation time, but the ellipticity of the outer contour decreases with the increase of inlet flow rate and relaxation time. In the gas-assisted extrusion process, due to the uniform distribution of radial velocity, shear rate and first normal stress difference of melt inside and outside the die, which tends to zero, the swelling deformation of the inconsistency between the catheter and die cross-section is effectively eliminated. It ensures the roundness of the catheter contour, main and secondary cavities, and it is independent of melt flow rate and relaxation time. It can effectively improve the quality of multilumen microtube extrusion forming and achieve precision molding of multi-lumen microtube.