A class of circularly-polarized cavity-backed slot antenna arrays with simplified feeding structure are proposed. Power dividing network is an essential feeding structure for the design of conventional antenna arrays, which results in complicated antenna structure. A novel feeding technique based on the electric field of cavity modes is introduced to feed the array elements. The slots on the top wall are directly fed by the electric field without additional feeding structure. Crossed slots with different lengths are introduced to achieve the self-phased CP radiation as they produce different perturbation on the two degenerate cavity modes. Therefore, high efficiency and simple antenna structure of the CP slot antenna array are achieved. Then, two CP slot arrays with 3 × 3 and 8 × 8 elements are presented to show the designed feasibility. Finally, the 3 × 3 CP antenna array is fabricated and measured to validate the design concept, which can achieve 12.6 dBic gain, 94% total efficiency, 80% aperture efficiency, and −25 dB cross polarization.