In this letter, a bandwidth-enhanced high-gain full-metal cavity-backed filtering slot antenna array is proposed. By replacing one radiating slot with 2 × 2 radiating slots, the enhanced gain is obtained without enlarging the antenna size. Besides, two cavity modes, i.e., TE101 and TE301, are combined to obtain an enhanced bandwidth. Metal ridges and metal posts are employed to make the two modes close to each other. Due to a similar radiating field of the two specific cavity modes under the proper perturbation and placement of the 2 × 2 radiating slots, a high gain over the enhanced bandwidth is obtained. In addition, the high-quality factor of the two cavity modes can inherently produce a filtering performance in terms of the realized gain and efficiency. The final measurement shows that the proposed slot antenna array can achieve 6.2% bandwidth, 97% total efficiency, and 10.4-11 dBi stable realized gain with a small aperture size of 0.84λ2. Good agreement between the simulation and measurement has well validated the proposed design concept.