The invention discloses a collagen micro fiber and a preparation method thereof. Provided are the collagen micro fiber and the preparation method thereof. The method comprises the following steps: 1) taking an isolated mammalian dermis and/or subcutaneous tissue layer as a raw material, and removing non-collagen components in the raw material to obtain a crude product; and 2) removing non collagen in the crude product obtained in the step 1), to obtain the collagen micro fiber. The preparation process of the method is simplified, and introduction of complex production equipment is not needed, so that difficult control of microorganisms, heavy metals and other factors in the production process is avoided, and the requirement of industrialized production can be met; the collagen micro fiber is suitable for large-area diffuse hemorrhage, has he advantages of fast hemostatic speed, high hemostatic efficiency, qualified heavy metal content and the like, and has wide application prospects.