In this paper, based on the theory of mode-matching and the coupled wave theory, A W-band TE62 mode generator by using waveguide mode transformation is presented. Because of the eigen-mode selection of coaxial waveguide, a TE1 mode in standard rectangular waveguide is coupled into a coaxial waveguide to excite a TE61 mode by an aperture. A transition follows on it to change the coaxial waveguide into a circular one. Finally, TE61–TE62 mode converter is achieved by using a periodic radius perturbation in circular waveguide. Calculation and analysis of the relationship between the mode conversion efficiency and structure parameters of waveguide also are finished. The validity of this study is confirmed by using electromagnetic simulation software. The conversion efficiency of TE62 mode reaches 94.5% at the center frequency 95 GHz, and the mode purity reaches 98.16% The bandwidth of the efficiency above 85% reaches 1 GHz, which can meet the demand of the high–frequency cold test of gyrotrons.