On the basis of Y loop elements, a new method to set circular slots on the periodic element to improve the narrow bandwidth of a Frequency Selective Surface (FSS) structure was presented. The FSS structure was calculated by Galerkin’s method in the spectral domain when the optimum radius was set to be 0.5 mm and the number of the circular slots to be 12, and the transmittance in the resonance frequency of 10 GHz has increased by 0.11 dB. The plane sample was fabricated by depositing coatings and a lithograph technology,and obtained measured values are in good agreement with the calculated values. The results show that the transmittance of this structure is about -0.62 dB at the resonance frequency,which has increased by 0.21 dB under the perpendicularity incidence compared with that of the Y loop without circular slot. Furthermore,the transmittances at TE 30°and 45°incidence are -0.66 dB and -0.81 dB, respectively,which has increased about 0.32dB and 0.27dB compared with those from the Y loop without circular slots.The bandwidth difference between the two structures is negligible, which are about 1 GHz and 0.8 GHz under TE 30°and 45°incidence, respectively. These results indicate that this method is an effective way to improve the transmittance of large periodic FSS structure at the resonance frequency.