In order to analyze the influence of dielectric truss structure on frequency selective surfaces (FSS) transmission characteristics, as an example, the FSS of Y ring unit and the truss made of polyimide are investigated by the finite difference time domain method. The relevant physical model is developed and pore blocking rate is defined. Through the analysis it is proved that the variations of truss period and rib width will affect the FSS, so they become the main parameters to measure dielectric truss structure, and that the improvement of the aperture stop ratio will increase transmission loss. When truss period changes from infinity to 80 mm the transmittance of passband is reduced by more than 0.9 dB on an average; when the ribs width increases from 0 to 10 mm, the transmittance of passband is reduced by more than 0.6 dB on an average. When aperture stop ratio is lower than 12.11%, the passband produces no shipt. When aperture stop ratio is less than 4.12%, the influence of the truss on FSS is negligible. The FSS specimens is fabricated by coating and lithography process, the dielectric truss is machined by numerically controlled machine, and the microwave measurement is carried out in a dark room. The experimental results and the calculation results are verified to be in good agreement with each other. Therefore the present study presents an experimental and theoretical reference for designing the FSS stealth radar with the dielectric truss structure.