Directional backlight plays an essential role in creating high-performance light field three-dimensional display systems. However, state-of-the-art directional backlight technologies are constrained by tradeoffs between directionality, uniformity, and thickness of the system. In this paper, we presented an ultra-slim directional backlight system and built a prototype to experimentally validate the design. The spread angle of the backlight system equals 3.41°, the illuminance uniformity of the backlight system is 92.85%, and the thickness of the backlight system is only 4.05 mm. The experimental tests show the effectiveness of this method in designing large-format ultra-slim directional backlights with exceptional directionality and illuminance uniformity.