Abstract: We designed a frequency selective surface(FSS) structure which can reduce the radar cross section(RCS)in band of waveguide slot antenna.The structure utilized the artificial magnetic conductor(AMC)loop as the basic unit, which were formed in helical arrangement on FSS reflection board and took the waveguide slot antenna as the bottom feed.In order to prove that the antenna which has good performance in RCS reduction, the same size of metal board is designed to compare with the reference antenna.The simulation results show that, when the wave vertically incident into the antenna, the RCS of antenna can achieve -25 dB, and the antenna maintains good reflective performance and normal radiation performance as well.