A high efficiency Denisov-type quasi-optical mode converter for a 170 GHzmode gyrotron is presented. The mode converter comprises a dimpled-wall launcher and a mirror system. Based on the coupled mode theory， the advanced launcher having two stages of perturbations is investigated. A mirror system of the converter is optimized and designed by using the vector diffraction theory. Simulation results show that the good Gaussian mode is converted from the circular waveguide mode and the mode conversion efficiency of a quasi-optical mode converter is 93.7%.