Abstract: In quasi-optical technique, it is convenient to use analytical formula to design the quasi-optical system. A quasi-optical system composed of lens and ellipsoidal reflectors designed by the analytical formula has good agreement with the actual results. However, there is obvious error in deducing the transformation properties of Gaussian beam reflected by a parabolic mirror with the analytical formula. That means it is not satisfied with the requirements of designing quasi-optical system composed of parabolic mirrors. Therefore, a large number of simulations about the transformation properties of Gaussian beam reflected by a parabolic mirror in the Ka-band are carried out with CST, and the analytical formula is revised with the help of the simulation results. The calculation results given by the revised formula is in good agreement with the results of the full-wave simulations about the transformation properties of Gaussian beam reflected by a parabolic mirror in the Ka-band and the W-band. As a result, this modified theoretical formula can be used not only to design the quasi optical system, but also to eliminate the long time-consuming computation by using full-wave simulation software.