In order to achieve Q-switched pulse output of the CO2 waveguide laser, a half-external cavity laser is necessary to insert the Q switch in the resonator. We study the relationship between the output power of the rectangular waveguide half-external cavity CO2 laser with the waveguide aspect ratio m=2 and the optical structural parameters of the resonator. We compare the output powers of the laser whose waveguide port clings to the plane mirror, and obtain the relationship between the loss generated by the resonant cavity mode coupling to the waveguide port and the radius of the total reflection mirror R, and the distance between the total reflection mirror and the waveguide port d. Based on the Fresnel diffraction integral theory, the relationship between the coupling efficiency of the resonator and R, d when m =2 is calculated with the Simpson algorithm variable step length theory. The results show that d=R and d=10 mm are two optical and structural parameters which are suitable for the rectangular waveguide half-external cavity CO2 laser with a high aspect ratio, where larger R will induce higher coupling efficiency.