Abstract: This paper presents a novel design of sub-reflctor radome for suppressing high power microwave (HPM) air breakdown in the HPM propagation of dual reflector antenna system. The sub-reflector radome based on composite material is an ellipsoid spherical shell filled with SF6 gas to increase air breakdown threshold. Firstly, the minimum outline of radome is obtained by calculating air breakdown region near sub-reflector with high power excitation . Then low-loss design method has been investigated, for one thing, the outline curve is optimized by using particle swarm algorithm to keep the angle between electromagnetic wave and the radome normal line less than 44 degree, and for another thing, thickness parameters of three layers are optimized to get maximum transmission coefficient. Furthermore, in order to prevent the difficulty of molding process, the radome is simplified as reinforced PMI hoop structure, which has low insert loss as well as good mechanical performance. Finally, the sub-reflector radome is fabricated and measured, the measurement results show the insert loss is less than 0.3 dB and the microwave transmittance is up to 93.4%, which means it affects the radiation performance little.