The invention provides applications of an agar oligosaccharide in cosmetics. The agar oligosaccharide has the structural molecular formula disclosed in the specification. The invention particularly provides applications of a degradation product of agar extracted from artificial red algae, such as purple laver, agar weed, Gracilaria or the like, in manufacturing cosmetic additives. The agar oligosaccharide can be used for preserving and absorbing moisture of the skin, and whitening the skin, and also can be used as a moisturizing agent, a moisture absorption agent or a whitening agent in cosmetics.