A graft copolymer was synthesized by means of radical copolymerization of degraded starch, phloroglucinol(PG) and sulfanilic acid (SA) and the reaction was catalyzed by horseradishperoxidase (HRP)/H2O2. It was discussed that the effectsof monomer composition, the temperature and system pH on the graft copolymerization. The ideal copolymer was synthesized in the pH 7.0 at 30°C when the molar ratio of PG and SA was 5:6. The structure of the copolymer was characterized by Fourier Transform Infrared (FTIR) spectroscopy. Meanwhile, the graft copolymer has been used in tanning process. The retanned leather exhibits excellent increase ratio in leather thickness and good dyeing ability which indicate that the graft copolymer has retanning and assisting dyeing properties.