The quasi-optical mode converter is the main part of the high-power millimeter wave gyrotron. The converter can separate the electron beam and electromagnetic wave，reduce the reflecting microwave into the high-frequency cavity，and realize the output of transversal Gaussian beam. The Denisov launcher for 140 GHz，TE22.6-mode gyrotron has been analyzed and designed according to the mode coupling theory. And the reflecting mirrors have been designed with the geometrical optics theory and optimized by Feko simulation. According to the calculation results，the conversion efficiency of electromagnetic energy is 97.3% and the Gaussian content of output wave-beam is 93.3% for the quasi-optical mode converter，which can meet the requirements of the gyrotrons.