A modified collagen fiber preparation method and application are provided. The modified collagen fiber is prepared by modifying a collagen fiber with a plant tannin; and a method of the preparation includes: mixing the plant tannin with the collagen fiber in a liquid environment with a pH of 5 to 8 to allow a reaction, and washing and drying a product. In the present disclosure, a plant tannin rich in phenolic hydroxyl can be combined with a collagen fiber in various ways such as multi-point hydrogen bonding and hydrophobic bonding, such that the plant tannin structure is introduced into a natural multi-layer micro/nano-structure of the collagen fiber; and due to a large number of phenolic hydroxyl structures in the plant tannin, the collagen fiber introduced with the plant tannin structure shows improved compatibility with a waterborne resin, and can produce strong hydrogen bonding with polar groups in the waterborne resin.