Abstract: The principle and design method of Denisov-type quasi-optical mode converter is investigated in detail. The operation process of the Denisov-type launcher is analyzed by applying the geometrical optics, and the Gaussian-like field distribution achieved on the waveguide wall is also derived. The method for designing a rippled-wall launcher is proposed on the basis of coupled mode theory. A simulation code for Denisov-type quasi-optical mode converter GQOMC-D is developed based on coupled mode theory, vector diffraction integration and physical optics, which is compared to the design parameters and experimental results reported in literature for its validity. According to this code, a Denisov-type quasi-optical mode converter used in 110 GHz TE22,6 mode gyrotron oscillators is designed. Simulation results indicate that a Gaussian-like beam is obtained at the output window with a scalar content of 98.4% and a conversion efficiency of 94.7%.