The quasi-optical transmission characteristics of high-order mode of high-power gyrotron oscillator are analyzed based on geometrical optics and coupled-wave theory, and the simulation code of quasi-optical mode converter with perturbation waveguide is compiled. The design method and results of Denisov launcher in 140 GHz gyrotron oscillator are given. The power distribution curves of each mode and the current density distribution on the waveguide wall are analyzed. The launcher structure with disturbance length of 60 mm, total length of 136.5 mm and conversion efficiency of 99.1% are obtained. And the results fulfill the requirement of 140 GHz gyrotron. The software of Feko is adopted to analyze the reliability of the results from numerical calculation, which lays technical foundation for the gyrotron in the application of thermonuclear fusion.