Abstract: This paper presents the design of a miniaturized ultra-wideband feed horn, which adopts an open-boundary quad-ridged horn structure. The backshort cavity at the feeding section is inspired by the Vivaldi antenna's reflective cavity design. By enlarging the cavity, the circulation path for surface currents at low frequencies is extended, which improves low-frequency radiation performance, thereby achieving a reduction in size. Compared to conventional quad-ridged horns, the proposed design reduces the size by approximately two-thirds to 0.45\lambda_0\times0.45\lambda_0\times0.35_\lambda , where \lambda_0 is the wavelength at the lowest operating frequency. The ultra-wideband feed operates at 2.2-16 GHz. Test results indicate that, besides a deviation near 6 GHz caused by assembling errors, the reflection coefficients at both ports remain below -8 dB across the entire operating band. Simulated results show that the aperture efficiency of the antenna exceeds 40% throughout the operating bandwidth. This feed horn is suitable for space science applications with strict requirements on size and weight.