Broadband double-ridged horn antenna with high-intensity out-of-band rejection is proposed. First, two pairs of outer wall of the horn and double-ridged waveguide are used to form an open-ended waveguide. The high-intensity out-of-band rejection is achieved by utilizing the high frequency transmission of waveguide. Then, an asymmetric stepped double-ridged structure is designed to obtain the desired impedance bandwidth by reducing the cut-off frequency of the horn’s main mode. The simulated results reveal that good impedance matching and stable radiation patterns are obtained from 12 to 18 GHz with 0.5 dB insertion loss in the passband. Meanwhile, transition bands from the passband to the stopband are narrow. The transmission coefficients below 11 GHz could be suppressed under -11 dB. All the results demonstrate that the proposed double-ridged horn antenna can realize high-intensity out-of-band rejection while maintaining well impedance bandwidth and stable radiation patterns in the passband.