With the continuous development of electromagnetic detection systems, the working frequency of radar gradually expands towards the low-frequency band such as P- and L-band, and the low-frequency horn antenna has also attracted more and more attention. Given the problems of miniaturization, lightweight and ultra-wideband design of low-frequency horn antenna, this paper presents the optimal design for the feed balun, ridge curve, back-feed cavity and sidewall of the traditional horn antenna, and proposes a performance-enhanced double-ridged horn antenna covering 0.21-2.3 GHz. In the entire working band, the antenna has stable radiation patterns and good port-matching characteristics; its VSWR is less than 2 and the main lobes of patterns do not split or deteriorate. The proposed horn antenna can be used as a standard calibration or measurement antenna in antenna or material measurement systems.