Abstract: In this paper, the theory of mode-matching technique(MMT) and optimization algorithm are used to develop a triple frequency multi-mode horn for the 110-meter antenna of Xinjiang Observatory with a working band of 4-12 GHz. This horn is composed of 225 circular waveguide steps, which is easy to manufacture. Compared with traditional multi-mode horn and corrugated horn, it has a prominent advantage of equating radiation pattern in ultra wide band. This horn is suitable for high efficiency feed for ultra wide band radio astronomy observation antenna or for other antenna designed for ultra wide band.