Abstract: We examined carefully the reliability of C8 code according to 3D Fresnel integral equations．　The　transmission behavior of the short wavelength laser in virtual confocal cavity with circular mirrors of spherical surfaces were computed and analyzed. Under condition of the same transmission distance, the distribution of farfield intensity changes from the projection of nearfield intensity distribution to the distribution of superGaussian when magnification on m changes from 10 to 1.42．