A plastic micro-tube is extremely small, and the melt is still in a molten state when the melt exits the die. The inner layer gas still flows in the micro-cavity after leaving the die, therefore, the diameter of the micro-tube increases gradually. When the outer layer gas leaves the die, it blows directly to the outer wall of the micro-tube. The flow of the double layer gas has a great influence on the extrusion of the micro-tube. To this end, a double layer gas-assisted extrusion model based on gas/melt two-phase flow is established. It focuses on the overlay effect of the double layer gas flow on the forming micro-tube. Through a finite element numerical simulation of the micro-tube extrusion process, forming the double gas layer inside and outside the tube wall, respectively, we obtain the shape, velocity, pressure drop and first normal stress difference of the micro-tube. The analysis shows that the double gas layer inside and outside the tube wall have asymmetrical effect on the melt, and they must be analyzed at the same time; the first normal stress difference is generated at the entrance of the die, exit of the die and downstream of the die exit; it reflects the extrusion deformation of the gas to the micro-tube, the degree of extrusion on the micro-tube wall, the distribution of the velocities X and Y, and the distribution of the pressure drop, to some extent. Compared with the micro-tube gas-assisted extrusion experiment, when the gas pressure is large, the result is consistent with the phenomenon of irregular corrugation on the wall surface of the micro-tube, the wall at the exit of the die is rapidly thinned and the wall downstream of the die exit gradually thins.