The invention discloses a polymer-coated aramid fiber and a preparation method thereof. The preparation method comprises the following steps: 1, mixing an inorganic salt with an inorganic alkali solution, and diluting with water to prepare a buffer solution; 2, placing an aramid fiber in the buffer solution under the protect of nitrogen, adding a solvent, horseradish peroxidase and an acrylic acid monomer, adding hydrogen peroxide drop by drop, and carrying out washing and vacuum drying after finishing a reaction to obtain the polymer-coated aramid fiber. By adopting above technical scheme of the invention, the enzyme inactivation rate is low, and the surface of the aramid fiber has a good swelling effect in a reaction medium, so the full contact of the biological enzyme and the fiber is guaranteed, and the grafting rate is improved; and the formed coating which contains active groups allows the cohesiveness of the fiber with other materials to be effectively improved, so a large space is provided for the further modification of the fiber. The technical scheme adopted in the invention has the characteristics of low damage to the fiber, mild reaction condition, simple and easily implemented technology, environmental protection, and high benefit of the industrialization production.