Abstract: A quasi-optical mode converter for high power gyrotron, consisting of a Denisov launcher and four mirror reflectors is designed and studied. Applying the geometrical optics, the operation mechanism of the Denisov launcher is analyzed in detail and the Gauss-like field distribution achieved on the inner wall is derived. The method for designing a dimpled wall launcher is proposed based on coupled mode theory. As an example, the launcher and mirror parameters are selected and optimized for a 110 GHz, TE22,6 mode gyrotron. Finally, a simulation code is developed to calculate the field distribution at aperture or on the mirrors. Numerical result indicates that a Gaussian-like beam is obtained through the output window, which scalar content is 95.8% and the conversion efficiency of the system is 94.7%.