An improvement equivalent circuit method (ECM) for analyzing finite and curved frequency selection surface (FSS) is proposed in this paper, double-square-loop FSS has been used in the curved FSS and the transmission coefficients of both TE and TM models have been analyzed by the ECM and full wave method. It has been shown that the improvement ECM has saved computational time in designing curved FSS, and has verified its feasibility compared with the full wave EM method.