The invention provides an optical thin film and a backlight unit. The optical thin film comprises a plurality of stacked transparent material layers, wherein a first composite structure layer is arranged between at least two adjacent transparent material layers and comprises a first bulge structure and a second bulge structure covering the first bulge structure, the first bulge structure comprises a plurality of first bulge parts sequentially arranged in the a direction, the second bulge structure comprises a plurality of second bulge parts sequentially arranged in the a direction, the first bulge parts and the second bulge parts penetrate through the transparent material layers in the b direction, the a direction and the b direction are perpendicular to each other and are parallel to the extending surface of each transparent material layer, each second bulge part covers at least one first bulge part, an air channel is formed by a gap between every two adjacent second bulge parts, and the first bulge structure and the second bulge structure have different refractive indexes. The optical thin film has good shielding performance, and the probability of occurrence of phenomena of moire interference, newton ring and the like is lower.