Abstract: As one of strong scattering sources of the aircraft head, RCS calculation methods of the inlet are usually based on ray tracing theory. But when solving large S bend, aspect ratio and incidence angle, these methods can produce big errors. The analysis of iterative physical optics method, with FMM and MPI is used to increase processing capacity. On the 32-node computing platforms, RCS of the real size inlet in X-band is achieved. The results agree well with those obtained by model method. It shows that the method can estimated the true inlet RCS.