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）.