In order to meet the requirements of high-power output and high-efficiency transmission of the 170 GHz gyrotron for fusion， it is necessary to convert the operating mode of TE_25，10 to a Gaussian beam for output. In response to this technical requirement， based on the vector diffraction theory， geometric optics， coupled wave theory， and phase correction algorithm， a 170 GHz， TE_25，10 mode high-efficiency quasi-optical mode converter was designed using a numerical calculation program. The mode converter consists of a Denisov launcher， a parabolic mirror， a quasi-elliptical mirror， and a phase correction mirror. Using Surf3D software and commercial simulation software FEKO for simulation calculation， the energy conversion efficiency of the designed quasi-optical mode converter is about 95.2%， the beam scalar Gaussian content is about 97.6%， and the vector Gaussian content is about 91.8%， reaching the application of gyrotron requirement.