The scattering characteristics of various rectangular coaxial line discontinuities, with which a satellite beamforming network (BFN) was built, were investigated by the finite difference time domain (FDTD) method. The scattering parameters were directly calculated through the simulation procedure of the wave propagation, thus the analysis was tremendously simplified as compared with the mode matching method. Good agreement was found between the numerical results and the experimental data. The effectiveness and accuracy of the present method were thus justified.