Abstract: Based on the analysis of the existing electromagnetic numerical calculation method, a fast radar cross section(RCS) calculation method is designed for two-dimensional planar array through extrapolation of sub-array, which combines the moment method and the Fourier series fitting in signal processing. This new method can be used for the RCS calculation of the periodic arrays covered electrically large aircraft. The efficiency of the algorithmhasincreased by nearly 25 times, compared with the efficiency of the traditional electromagnetic business software FEKO, and then the efficiency and correctness of the algorithm are verified. By comparing the measured data with the simulation data, the main lobe and surrounding sidelobe of the RCS curve are basically bonded, and then the feasibility of the algorithm is verified.