Abstract: In view of the increasingly important radar antenna frequency selection surface (FSS) on military equipment, the characteristics of the FSS radar cross section (RCS) area and the difference from the metal surface RCS are analyzed, and the internal mechanism of this difference is clarified, which provides the theory guidance and solution for the improvement of the FSS radome and the similar cavity RCS Method. Firstly, the generation mechanism of the radar cavity RCS covering the frequency selection surface is theoretically analyzed, and then the data of different radar cavity RCS are calculated with the full wave rigorous simulation algorithm, and the theoretical analysis conclusion of the front is authenticated with the internal field map. The theoretical and simulation analysis shows that the high reflection characteristics outside FSS are not fully reflecting electromagnetic signal as the metal surface. The transmission of the radome and the electromagnetic wave energy accumulation and the two radiation caused by the cavity effect in the mask must be considered. These factors have great influence on the RCS of the frequency selective radar cavity. Finally, a method of reducing cavity resonance effect and energy converging to improve the frequency selection mask RCS is proposed, and the method makes the improvement of RCS up to 10 dB in most frequency bands, which has achieved good results.