The quasi-optical mode converter consists of a ripple-wall waveguide launcher, a quasi-elliptical mirror and two toroidal mirrors.The wave beam in the launcher can be described by geometric optics. the coupled-mode theory has been used to analyze the operation of the prebunching waveguide launcher. 170 GHz TE22,6 mode was transform into 8 sattlite modes by the ripple-wall waveguide, and a quasi-Gaussian field was obtained by superposition of the main mode and satelite modes. Then the radiation was launched from the prebunching section by cutting the waveguide wall around a gaussian bundle.The launcher length is less than 150 mm, and the quasi-Gaussian bundle can be used directly.