The present invention relates to a method for preparing an ultra-low-molecular-weight keratin peptide and a use thereof. In particular, the present invention relates to: an ultra-low-molecular-weight keratin peptide preparation method using the cultivation of microorganisms having keratin decomposition activity in a culture medium including keratins, ultrafiltration, ion exchange chromatography, and gel filtration chromatography; a peptide prepared according to said method; and cosmetic and food compositions for preventing or reducing skin aging or skin wrinkles, comprising said peptide. The keratin peptide preparation method according to the present invention enables environmentally friendly biological treatment of waste resources and effective refinement and recovery of an antiaging, functional, ultra-low-molecular-weight keratin peptide. In addition, the keratin peptide according to the present invention has inhibitory activity against the expression and activity of MMP-1, which is an enzyme that causes skin aging by decomposing collagen, thus has excellent effects of preventing skin aging and reducing skin wrinkles, is appropriate for use as cosmetic, pharmaceutical, and food compositions for preventing, reducing, or treating skin aging and skin wrinkles by having no toxicity to skin cells, and thus can be usefully used for efficient and quick production and development of high value-added functional cosmetic materials.