A double screen FSS with positive reflection phase gradient(the general with negative slope),above 0.83 reflection magnitude value and partially reflection features at 5.42~5.80GHz is designed and fabricated, the new waveguide antenan bandwidth is broadened, gain is improved and directivity is enhanced by applying the double screen FSS as a radome on the radiation orientation.The FSS radiation enhanced and scattering reduced mechanism are analyzed by adopting ray theory and Fabry-Perot resonator model, and the capabilities of new antenna are validated by simulation and experimentation. The results show that: the bandwidth of the antenna is increased by 34.6% and the gain of the antenna is enhanced by 6.94 dB, and the half-power beam width of E-plane and H-plane are reduced by 240 and by1280 respectively.