Abstract: An analysis of waveguide branch-line directional coupler,whieh is used on spacecraft,is given in this paper using network theoryand mode match method.By taking into account of the high order mode coupling,the calculationresult is very coincident with the test data of an experimental sample.Therefor the method given here provides a powerful tool for the design ofwaveguide branch-line directional coupler,and can be directly used for de-velopment of the BFN of Ku band shaped beam antenna system of communi-cation satellite and for the areas where the waveguide coupler with dif-ferent couple coefficient such as various waveguide beamforming networkis used。