Composite microtubes are hollow conduits composed of two or more materials, characterized by their small size and multifunctional integration. They have a wide range of applications in medical, fiber optic tubing, intelligent robots, and other fields. The numerical model was established for the coextrusion flow of polymer double-layer composite microtubes and finite element method was applied for stable solution. By analyzing the distribution of physical field quantities such as melt velocity, pressure, and shear rate at different wall thicknesses, the influence and mechanism of wall thickness on extrusion swelling and interface position of composite microtubes were studied. The research results show that in traditional extrusion process, when the wall thickness increases from 0.20 mm to 0.50 mm, the extrusion swelling rate, pressure drop, and shear rate of composite microtubes decrease, but the interlayer interface fluctuation increases. The extrusion swelling rate decreases from 55.3% to 45.9% , and the interface migration increases from 0.01 mm to 0.03 mm. Therefore, increasing the wall thickness has limitations on improving microtube quality. In gas-assisted extrusion process, the composite microtubes with different wall thicknesses exhibit relatively stable performance, with no significant differences. This means that the gas-assisted technology enhances the stability of extrusion process for composite microtubes with different wall thicknesses and improves product quality.