In order to improve the filtering characteristics of Frequency Selective Surfaces(FSSs) and to realize their applications to bandpass radomes, a thick-screen FSS with circle slot elements filled with dielectric materials was designed and a corresponding experimental FSS sample was produced.In theoretical analysis for the thick-screen FSS, an equivalent problem was established,then the Floquet’ s theorem was employed to expand the field from the infinite array elements into an infinite number of plane wave.Furthermore,the boundary condition was enforced at aperture boundarys,and the method of mements was used to solve the equivalent magnetic-field integral equation.A test was carried out in a microwave darkroom，and measured results show that it is in good agreement with calculated ones.The results show that the center frequency mainly depents on interelement spacings, incidence angles and dielectric permittivities.When three parameters increase from 30 mm to 36 mm, 10° mm to 30° mm, 2.5 mm to 7.0 mm respectively,the center frequency can be reduced by 2.1, 1.5 and 1.9 GHz correspondingly.Moreover, the bandwidth is related to the shape of the element.When the diameter of element changes from 14 mm to 17 mm,it increases 1.2 GHz.It is concluded that improving transmission characteristics of thick-screen FSS is a task on optimizing various parameters.It can obtain optimal transmission properties by choosing filling dielectrics and other parameters appropriately.