Since the transmission system of High Power Microwave(HPM) cannot realize self-tracking, the exploration of adding a TE21-mode coupler into the HPM system is conducted. As the existing TE21-mode couplers have the problems of power capacity, an X-band GW-level TE21-mode coupler is proposed to explore the application in the HPM system. The simulation results demonstrate that the coupling coefficient of TE11-mode of the designed TE21-mode coupler is less than -40 dB from 9.0 GHz to 10.0 GHz, and the coupling coefficient of TE21-mode is larger than -0.5 dB. The power capacity of the coupler is on GW level.