Based on a Y loop element, a new method which sets circular slot on the periodic element to improve frequency selective surface (FSS) structure of narrow bandwidth and high transmittance is presented. The FSS structure is calculated by Galerkin′s method in spectral domain. The optimum radius is 0.4 mm, and the numbers of the circular slot is 12. The resonance frequency is 15 GHz, and its transmittance is increased 0.12 dB. The plane sample is fabricated by depositing coatings and lithography. The agreement between the calculated and measured values is good. Compared with the Y loop without circular slot, the results show that the transmittance of this structure which is about -0.69 dB at the resonance frequency increased 0.16 dB under the normal incidence. The transmittance at TE 30° and 45° incidence are -0.64 dB and -0.78 dB, respectively. Compared with the Y loop without circular slot, the transmittance are increased about 0.345 dB and 0.31 dB, respectively. And the bandwidth difference between the two structures is negligible, and they are about 1.26 GHz and 0.97 GHz under TE 30° and 45° incidence, respectively. So this method is an effective way to improve the transmittance of large periodic FSS structure at resonance frequency.