The design method utilized to small coupling Leaky-Waveguide Antenna(LWA) cannot be applied directly to the large coupling LWA. CST is adopted to extract the leaky-wave structure’s electrical parameters for designing a leaky-waveguide antenna with large coupling, and the design principle and the related physical parameters acquisition method of the high power rectangular LWA are theoretically analyzed. A 2.20 m high power rectangular LWA working in L band is designed and simulated by CST software, the results show that the gain of the antenna is 15.8 dBi and the main lobe direction is matched well with the theory. Moreover, the power handling capacity is about 1.6 GW, which provides a good choice for high power microwave application.