In this paper, a Quasi-Optical launcher operating at 263 GHz with low order TE_3,2 mode is designed for THz gyrotron at Wuhan National High Magnetic Center (WHMFC). A TE_3,2 to TE_8,3 mode converter is designed up front, because the lower-order operating TE_3,2 mode does not follow the mode selection rules for Denisov-type launchers. The launcher consists of a left-handed (LH) TE_3,2 to right-handed (RH) TE_8,2 11-fold helically corrugated waveguide mode converter, a TE_8,2 to TE_8,3 azimuthally symmetric ripple-wall waveguide mode converter and a Denisov-type launcher. A coupled-wave theory MATLAB code for analysis of the electromagnetic fields at the deformed waveguide wall has been developed. The commercial simulation result shows the launcher can generate a Gaussian-like mode with a scalar correlation coefficient of 93.50% and a vector correlation coefficient of 85.65% on the observation surface.