The invention provides an ultra-wide-band horn antenna comprising a mode conversion feedback cavity part, a coaxial line excitation part, and a double-ridged horn part. The mode conversion feedback cavity part includes a feedback cavity, a short-circuit board, a cross bump, a wedge, a slope, an upper ridge, and a lower ridge. Through a metal sleeve, the short-circuit board, the slope, the wedge, a horn narrow side wall and other structures, the antenna has low voltage standing wave ratio and good radiation pattern. In the whole working frequency band of the antenna, the standing wave of the antenna is effectively reduced, and the gain of the antenna is improved. The horn narrow side wall is composed of a position-adjustable metal gate, so that the horn antenna has good low-frequency characteristic, the high-frequency radiation pattern is not deteriorated, and satisfactory impedance matching and radiation characteristics are obtained in the wide band from 0.8GHZ to 20GHZ. The ultra-wide-band horn antenna has the advantages of large power capacity, wide frequency band, high gain, and good directivity.