Abstract: The dihedral corner reflector with wing and fuselage joint of aircraft is a very strong electromagnetic source in a wide angular range. But traditional RCS computation method is only suit for the ideal dihedral corner reflector. This paper adopts Geometrical Optics (GO) and Physical Optics (PO) methods to compute double and thrice bounce contributions of a rectangle dihedral corner reflector. And on the base of illumination width, a concept of Equivalent Illumination Area (EIA) is presented. Through the analysis of EIA, a modified computational method is given, which can analyze a dihedral corner reflector with arbitrary shape of planes and with shadow objects. To valid this method, RCS of wing and fuselage joints of a certain hypersonic vehicle is computed, and the result validates its correctness.