The invention provides a method for preparing protein fillers used for tanning by taking hairs of animals as materials. The invention is characterized in that hairs of animals obtained by the protecting-hair depilation are cleaned by water and soaked in a 0.01 to 0.5m inorganic acid solution, which at least submerges the hairs, for 10 to 48 hours, and the hairs are washed by tap water until the hairs are neutral, and the hairs are drained and naturally air-dried; 600 portions by weight of the hairs are picked up and soaked in a reducing agent solution with the concentration of 2 to 30wt percent, which at least submerges the hairs, for 10 to 48 hours, and the hairs are drained and naturally air-dried; the pretreated hairs are put in a reaction kettle provided with a stirrer, a thermometer and a reflux condenser, and an alkali compound solution with the concentration of between 0.5 and 20wt percent, which at least submerges the hairs, is added, and the reaction kettle is heated to a temperature of between 50 and 95 DEG C and keeps the temperature for 2 to 20 hours, and the thick liquids are obtained, and the pH value of the thick liquids is adjusted to between 5 and 8 by an inorganic acid solution after the thick liquids are cooled, and the filtrate with the solid content of between 10 and 20 percent, which is rich in containing keratin polypeptides, is obtained, and finally the protein fillers for tanning are obtained after the filtrate is condensed and dried.