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.