Abstract: The overmoded circular waveguide can achieved high power capacity, which will inevitably lead to high order mode. In order to monitor the output power and frequency spectrum of the X-band long pulse high power microwave source, an X-band high power microwave broadband mode selection directional coupler was designed by the simulation of CST software, which is used to couple TM₀₁ mode and suppress the TM₀₂ and TE₁₁ mode. The sizes of waveguide and coupling hole were decided by the small coupling theory and phase superposition principle. At the central frequency range of 9.0-9.8 GHz, the simulation results show that the coupling degree of the coupler is about (-59.08±1) dB, the directivity of TM₀₁ mode is more than 30 dB, the suppression of TE₁₁ mode is more than 15 dB, and the suppression of TM₀₂ mode is more than 30 dB, the power handling capacity is 2.5 GW.