According to the application prospects of micro drone radar, precision guided weapon radar, and wireless communication terminal equipment, an asymmetric primary and secondary waveguide directional coupler is designed. The coupler uses an equally spaced porous coupling structure with different shapes of main and secondary waveguides, and the signal of the TE10 mode of the rectangular waveguide is coupled to the TE11 mode of the secondary circular waveguide. The isolated port achieves the effect of reverse cancelation, and good coupling and isolation can be obtained using the principle of phase superposition. The center frequency of the directional coupler and the relative bandwidth are 400 and 40 GHz, respectively. The results show that the coupling degree of the directional coupler is approximately -13.8 to -12.8 dB, which achieves a weak coupling effect and has a good coupling stability. The isolation is better than -24.5 dB, the through insertion loss is approximately -3 to -2.5 dB, and the performance is good.