In this article, a wideband cavity-backed slot array antenna with high gain and high aperture efficiency is presented. A novel cavity-backed subarray is designed with compact size and applied to construct a 4 × 8 array. The array antenna is composed of two feeding layers and a radiating layer. The subarrays are placed on the top layer and SIW structure feeding network on the two layers below is used to feed them. The presented antenna is designed, fabricated and measured. The measured results reveal that the working bandwidth reaches up to 17.6%, ranging from 18.7 to 22.3 GHz with the peak gain of 19.5 dBi. Besides, the maximum aperture efficiency of 77.6% is calculated by employing measured gain and aperture size at 21.9 GHz. The presented array can be utilized as a receiving antenna in satellite communication systems.