Abstract: To address the application requirements for high power capacity, high efficiency, and low-profile array antennas, this paper proposes a slotted waveguide antenna (SWA) element integrated with horn cavities and alumina ceramic-filled grooves. The theoretical relationship between slot width and power capacity in conventional SWAs is first derived, and a conventional SWA prototype was designed, fabricated, and tested. Measurements of the reflection coefficient, radiation pattern, and gain demonstrated good agreement with simulations, and power capacity tests confirmed that the designed antenna achieves no less than 2 MW, consistent with the value predicted from the derived relationship. The proposed SWA maintains the same aperture size as the conventional design while incorporating a horn cavity above each radiating slot and symmetrically distributed alumina ceramic-filled grooves along both sides of the waveguide’s broad wall, with only a 0.09λ increase in profile height. Simulation results show that the proposed structure increases power capacity by 1.7 times and significantly improves the aperture efficiency (AE) from 66.7% to 90.7%. A 4-element array based on the proposed element achieves mutual coupling (MC) suppression below –27.1 dB without additional decoupling structures, and attains a peak gain of 25.64 dBi at 2.458 GHz, corresponding to an AE of 96.1%. The proposed design thus enables co-optimization of gain, power capacity, and AE, demonstrating strong potential for high-performance low-profile array applications.