A new kind of dual-band horn antenna for S/X band radiation with high power handling capacity is proposed, which consists of coaxial feed structure flare angle horn and a polyethylene cover. Low band microwave is fed by TE11 coaxial waveguide mode, and high band microwave is fed by TE11 circular waveguide mode. Based on the specific structure of the horn antenna, mode matching method is utilized to determine scatter matrix of each discontinuity in cross section. To obtain the overall scatter matrix of the antenna, it is necessary to progressively cascade the scatter matrixes, and the overall scatter matrix determines the propagation properties of the horn antenna. Far field radiation pattern of the dual-band antenna can easily get by using mode expansion technology. Furthermore, a dual-band antenna at 3.6 GHz and 9.5 GHz is designed, and theoretical and numerical simulations agree well with each other. It reveals that the dual-band antenna has high power handling capacity with GWs level, and good farfield radiation pattern at each band has been achieved. The theoretical and numerical simulation investigations show that it is a promising method for realizing multi-channel multi-band HPM integrative radiation, which is significant for HPM in some special applications.