A dual-resonance wideband full-metal cavity-backed slot antenna array (CBSA) with high efficiency is proposed. A cavity mode and a resonant-iris mode are used to produce a wide impedance bandwidth. Radiation slots (elements) placed on the top wall are directly fed by electric fields of the modes using a single feeding slot without extra structures. This simplified feeding structure can reduce the antenna complexity, and can also improve the radiation efficiency by reducing power loss along with the conventional feeding power dividers. Thereafter, two wideband and high-efficiency slot arrays with 2 × 2 and 4 × 4 elements are designed based on these two cavity modes and proposed a simplified feeding structure. The measurement of 2 × 2 slot array shows that the proposed 2 × 2 slot antenna array can achieve 15% bandwidth, 13.4 dBi peak gain, and 95% total efficiency.