Abstract: In order to meet the requirements of practical interference test, an ultra-wideband high-gain double-ridged horn antenna with slot on the ridge is proposed. Firstly, the quadratic Bezier curve is introduced as the gradient curve of the ridge for the first time and the feasibility of the curve is verified, and the voltage standing wave ratio (VSWR) bandwidth of antenna is 2 GHz~18 GHz. Then,the problems of antenna high-frequency gain degradation and radiation pattern distortion are solved successfully by adding wedge-shaped blocks and adopting conical gradient ridge. Finally, the VSWR at low frequency band is further reduced by making tapered slots on the ridge. The simulation results show that the VSWR of the antenna is less than 1. 9 in 2 GHz~18 GHz and less than 1. 5 in 5 GHz~17 GHz. In addition, the high-frequency gain is stable above 18 dBi without loading the dielectric lens. Prototype measurement results show that the VSWR and gain of the antenna are well agreement with the simulation ones. The antenna can be used in antenna interference measurement systems and as a feed source of high gain reflector antennas.