The launcher is an important component of the gyrotron builded-in quasi optical convertor. The propagate characteristics of high order modes in volume waveguide are analyzed in this paper based on geometric optical theory. The waveguide coupling equations and the corresponding coupling coefficients of the modes in the perturbation waveguide are derived based on the mode coupling theory. The power distribution in the perturbation waveguide and the field on the wall are analyzed by the numerical calculation code which is written in MATLAB language. The 3D simulation software can be used to verity the numerical result. As an example, a Denisov launcher of 94 GHz, TE6,2 is designed, and the numerical simulation result shows that the conversion efficiency is more than 98%, The hot-test experimental results show that the profile of output field is well consistent with the numerical result.