The quasi-optical launcher is an important part of quasi-optical mode converter in high power gyrotron. The circular waveguide quasi-optical launcher was analyzed with the theory of geometrical optics. To form the Gaussian mode(TEM00) at the cut of the launcher, the coupled-wave theory was employed to design corrugated waveguide quasi-optical launcher. A high efficiency launcher has been designed for 140GHz, TE28,8 mode. Results show that total length of the launcher is only 205.2mm, and the cut length is 47.2mm. At the helical cut the scalar correlation efficiency of the Gaussian mode in the Brillouin zone is greater than 98%. The method can also be used for the designs of other frequency and mode gyrotron quasi-optical launcher.