As one of strong scattering sources of planes, assessing accurately the characteristic of radar cross-section (RCS) about inlets is still hot in investigation into computational electromagnetics. An iterative physical optics method, which an impedance boundary condition and forward-backward method are added to, is studied and a box-segmentation method of blanking off is proposed. It can compute accurately the electric magnetic current distribution on the wall and the RCS. What is more, by developing a kind of software which can be verified by the relevant examples, the RCS characteristic of any metal or dielectric cavity can be evaluated. On this basis, using the software, the numerical simulation about the metal/coated medium S-diffuser with rectangular shape is carried out, and the RCS variational regulation of diffuser under an incident wave length in 0.03 m is obtained. The improved iterative physical optics method is applicable to the design of any cavity and the study of coating absorbing materials, therefore the design cycle is shortened.