Abstract: In shared-aperture antennas operating at different frequencies, high-frequency antennas often suffer from severe scattering interference due to the close proximity of antennas across different bands. To address this issue, this paper proposes a broadband Frequency Selective Surface (FSS) transmission structure based on the multi-resonance effect and applied it in a shared-aperture antenna operating in the S/C band. By integrating the FSS wave-transmitting structure, the radiation of the high-frequency antenna is shielded from the influence of the low-frequency antenna while maintaining the performance of the low-frequency antenna itself. Simulation and experimental results verify that the designed broadband FSS structure exhibits excellent performance in the range of 3300-4200 MHz, effectively improving the radiation pattern of the high-frequency antenna elements. Moreover, within the operating frequency band of the high-frequency antenna, the FSS structure enhances impedance matching and boosts gain.