The chitosan was degraded by hydrochloric acid and the effects of concentration of chitosan, amount of hydrochloric acid, reaction temperature and reaction time on the viscosity of chitosan were investigated. The results showed that the degraded chitosan with low viscosity could be prepared when chitosan (wt 6%) was dissolved in acetic acid solution and hydrochloric acid (wt 3%) at 90°C for 6 h to decrease the viscosity from 7.52 mPa·s to 2.48 mPa·s. Then, a graft copolymer was synthesized by copolymerization of degraded chitosan with methacrylic acid (MAA) and acrylamide (AAM) by using horseradish peroxidase (HRP)/H2O2/acetylacetone (ACAC) as initiator. The structures of original chitosan, degraded chitosan and the copolymer were characterized by FTIR. The copolymer was applied to the leather industry as an amphoteric retanning agent. The applied results showed that the retanned leather had the merits of good uniformity fullness softness, strong selecting filling properties, good dyeing ability and antimicrobial activity.