An X-band multimode conical horn has been designed with three sections of varied flare angles. The results obtained by experiment measurements are in good agreement with the simulation results. E and H patterns of the conical horn are well equal in the 10 dB regime. The simulation results show that the 10 dB widths of E and H patterns are about 40°. The high power microwave(HPM) experiment results show that the 10 dB width of E pattern is 40°, and the 10 dB width of H pattern is 43°.The conical horn’s power capacity reaches 3 GW in the simulation. In HPM experiment, when the power reaches 1.3 GW,no breakdown phenomena appear.