Abstract: This paper focuses on a special kind of three-plate cavity reflectors which is useful in EM backscattering model of large complex targets over land or sea surface. After employing Computer Graphics (CG) method to eliminate shadowing effect, Physical Optics (PO) and Area Projection (AP) method are combined to analyze specular reflection and multiple scattering mechanism to calculate the Radar Cross Section (RCS) of different reflectors. Finally, by combining with sea reflection coefficient under different sea states, a procdure is proposed to compute the RCS of simplified ship targets over sea surface. The results show that multiple scattering can form obvious RCS characterization under certain conditions.