Abstract: Analysis and experiments were conducted to study how to fabricate large-area polyimide multimode waveguide by means of CO2 laser direct writing. Firstly, the laser fabrication model was built. Secondly, the relationship between the temperature distribution in the material and the laser scanning parameters was simulated. Finally,a method was proposed to increase the verticality of the sidewall of the waveguide. The influence of the processing parameters on the property of the waveguide material, the structure of the waveguide and the roughness on the surface of the waveguide was also studied by experimental method.The results show that the width, verticality and roughness of the waveguide depend on the laser power density, direct writing speed and the position of the aperture.