As one of the strong electromagnetic scattering source of the aircraft, the radar cross section (RCS) of the nozzle can be reduced by the special geometry and coating microwave absorbing material at the backward direction of the aircraft. In order to simulate the radar scattering characteristics of six coating models and one metal surface model of double S-shape nozzle, the mode combining impedance boundary condition with iterative physical optics (IPO) method is built. Forward and back-forward IPO method and open MP, MPI parallel computing technology are added to accelerate the convergence and reduce computational time. Besides, the ray tracing method is also adopted to improve the efficiency of geometric blanking judgment. The RCS variation regulations of 7 models under X waveband are obtained. The results indicate that coating medium can effectively diminish the RCS of double S-shape exhaust system. Proper coating method can not only reduce the RCS of exhaust system significantly, but also be economical, easy to coat and light-weight. Compared with all parts coating medium model, the model which is only coated at nozzle outlet can reduce microwave absorbing material cost by 73.6% and ensure that the maximal increment of RCS is less than 15.6%. Comparing with the metal model case, The RCS will decrease by at least 18.5%. The improved IPO method can be applied to the RCS evaluation of cavity and the study of coating absorbing material method, and provide technical support for the coating medium model experiment.