A wideband full-metal cavity-backed filtering slot antenna is proposed in this communication. The rectangular iris on a metal wall that inside a waveguide can be seen as a shunt LC resonant circuit, which can produce a resonance, called the resonant-iris mode. By cascading multiple layers of metallic walls with resonant iris, a bandpass filtering response can be achieved. And a filtering antenna can be realized when the resonant iris on the top layer of metallic wall is reused as the radiating slot. The coupling strength is easily controlled by modifying the distance between the adjacent resonant irises. Larger coupling strength can be obtained by reducing the distance between the two resonant irises, thus obtaining a wider bandwidth as well as a lower profile. For verification, a second-order and a fourth-order filtering antennas using the resonant-iris mode are designed, analyzed and simulated. A prototype of the fourth-order filtering antenna is fabricated, and the experimental results show that it can achieve 70% bandwidth with good filtering performance and stable radiation pattern. Besides, the total efficiency is higher than 90% over the entire operating band. Good agreement between the simulation and measurement has well validated the proposed design concept.