A terahertz quasi-optical mode converter with wide bandwidth was studied and designed. The quasi-optical mode converter used Denisov launcher, which had high efficiency, and worked in TE_6,2 mode. It was used to realize mode conversion in gyrotron. As the parameters of Denisov launcher were the main factors affecting the broadband performance of quasi-optical mode converter, the optimal design of the radiator parameters could increase the bandwidth of the mode converter. Using the self-developed quasi-optical simulation program for simulation, the bandwidth of the mode converter can reach 2 GHz (center frequency 94 GHz).