A dual-band structure reused antenna with quasi-elliptic bandpass feature is proposed in this paper. The antenna consists of a cavity-backed slot array working at K-band (24.2-26.1 GHz) and a frequency selective surface (FSS), which is transparent to the K-band radiation and also reused as a radiator at S-band (2.5-2.7 GHz). The FSS unit cells are designed based on cross-slot structure. By introducing asymmetric boundaries to each unit cell, the FSS obtains high frequency selectivity with two transmission zeros located on each side of the passband. The proposed antenna is fabricated and measured. The measurements have a good matching with the simulations.