This paper demonstrates an effective designing method of the high power rectan- gular waveguide directional coupler. The coupler designed using this method has wide band, high directivity, flat coupling, low voltage standing ratio and manufacturing accuracy. According to the required technology parameter, and the engineering design method based on the modified Bethe aperture coupling theory, the relevant parameters of equally spaced waveguide directional coupler are preliminarily designed. On the basis of this, using simulation software HFSS to do the simulation and optimization of the space between three outside coupling holes of the head end and the tail end, the performance of isolation is improved. A directional coupler at Ku waveband is designed in this paper, the error between the simulation value of coupling and the required value −10 dB is less than 0.6 dB, and after optimization, the isolation is less than −47 dB in the whole operating frequency band.