Ultrasonic-assisted brazing (UAB) technology has been introduced to prepare the diamond/AlSi composite coating on Ti-6Al-4V substrate. The microstructure and wear properties of the coating were studied. The results showed that AlSi filler metal had a certain spreading and climbing phenomenon on the diamond surface, indicating a significant improvement in wettability between AlSi filler metal and diamond by ultrasonic-assisted brazing. Under ultrasound action, the microstructure of AlSi brazing alloy was greatly refined, and the Ti atoms in the Ti-6Al-4V substrate diffused to the interface between diamond and AlSi brazing alloy and formed TiC on diamond particles. There was no thermal damage to diamond particles when the brazing temperature was 750 °C. UAB improved the microhardness of the coating alloy matrix due to the increasing quantity and distribution range of TiAl 3 as reinforced particles but had no obvious effect on the microhardness of the Ti-6Al-4V substrate. The wear resistance of Ti-6Al-4V substrate with diamond/AlSi composite coating was promoted due to the dense diamond particles, the formed TiC on diamond particles, and the increase in microhardness of the alloy matrix of diamond/AlSi coating under ultrasound action.