An effective method was proposed to design a high power coaxial-waveguide di- rectional coupler applied to the short-circuited line method. In this paper, the coupler designed using the proposing method has wide band, high directivity, flat coupling coefficient, lower SWR, and low requirement for machining accuracy. According to the required technology parameter, preliminary designed the aperture waveguide directional coupler and the adapter. In order to achieve the more excellent indices, we simulate and optimize the distance between the two hole and the two ends hole size of the coupler, the step size of the adapter. The VSWR of coaxial- waveguide transition designed in this paper is less than 1.06 in the 12.4–18 GHz octave bandwidth, and the high modulus produced is very small. Therefore, coaxial-waveguide directional coupler of the directivity and the coupling coefficient are improved.