To explore the influence mechanism of external gas-assisted pressure on the extrusion process of the plastic double-lumen catheters with a non-uniform wall thickness, the viscoelastic extrusion flow model was established. The numerical simulations were performed by the finite element method combined with the PhanThienTanner constitutive equation. The flow field distributions were systematically analyzed, including the melt velocity, pressure, shear rate, and first normal stress difference, at varying external gas-assisted pressures. The results indicate that when the internal gas-assisted pressure remains constant at 3000 Pa, the catheter shrinkage rate increases from 2.5% to 9.2% as the external gas-assisted pressure rises from 1000 Pa to 5000 Pa. Meanwhile, the ellipticity of the outer contour and the inner cavity exhibits a trend of decreasing firstly and then increasing, reaching its minimum at 4000 Pa. The increase in the external gas-assisted pressure significantly enhances the radial and axial velocities, pressure, and first normal stress difference of the melt. Notably, the first normal stress difference at the gas-melt interface plays a crucial role in causing the melt shrinkage deformation and flow field redistribution. The gas-assisted extrusion experiments validate the reliability of the simulation results and reveal that an excessively low external gas pressure ( less than 1000 Pa) hinders the formation of stable gas-cushion layer, while an excessively high pressure ( greater than 5000 Pa ) leads to excessive shrinkage, increased ellipticity, and even melt rupture. Therefore, the reasonable regulation of the external gas-assisted pressure, particularly within the range of 3000~4000 Pa, plays a significant role in optimizing the dimensional accuracy and morphological stability of the double-lumen catheters.