In this paper, a calculation method of the jet engine inlet on radar cross section (RCS) is introduced by means of mode superposition on the basis of the traditional waveguide mode theory. The key of this method is to separate a complex terminal into some simple terminals, and to calculate separately the waveguide mode reflectance and polarization scattering matrix of each simple terminal. Then, RCS of a complex inlet can be calculated by the mode superposition of reflectance and polarization scattering matrixes. The advantage of the method is that a complex mode can be easily piled up by many simple modes and various inlet modes can be superposed conveniently.