OBJECTIVE： To study the in vitro antioxidant activity of chitosan （CTS） degradation derivatives and its preventive effects on drug-induced liver injury fibosis. METHODS： Acid hydrolysis method was used to prepare the CTS degradation derivatives CTS-3， CTS-6， CTS-8， CTS-10 for different hydrolysis time （3， 6， 8， 10 h）. The viscosity-average relative molecular mass and deacetylation degree of CTS and its degradation derivatives were determined， and its antioxidant activity was evaluated by detecting its in vitro scavenging ability on 1，1-diphenyl-2-trinitrophenylhydrazine （DPPH） and superoxide anion （O2－） radicals. Using CTS-10 for in vivo liver injury fibosis prevention test， mice were randomly divided into normal control group （water）， model group （water）， CTS-10 high-dose， medium-dose， low-dose groups （100， 50， 25 mg/mL）， 8 in each group. Mice were intragastrically administrated 0.2 mL， then withdrawal after continuous 24 d. Then levofloxacin hydrochloride was intragastrically given for 7 d to establish drug-induced liver injury model （except for normal control group）. Western blot method was used to detect the expressions of tumor necrosis factor α （TNF-α）， transforming growth factor β1 （TGF-β1） and Decorin protein in liver tissue of mice. RESULTS： The viscosity-average relative molecular mass of CTS， CTS-3， CTS-6， CTS-8， CTS-10 were 21.70×104， 6.70×104， 6.30×104， 5.01×104， 4.87×104； and deacetylation degree were 83.44％， 74.62％， 67.28％， 64.83％， 54.23％， respectively. All of them had certain scavenging ability on DPPH and O2－， in which， CTS-10 was the strongest （25.47％ and 56.31％）. Compared with normal control group， expressions of TNF-α， TGF-β1 and Decorin protein in liver tissue in model group were enhanced （P＜0.05）. Compared with model group， expressions of TNF-α， TGF-β1 and Decorin protein in liver tissue in CTS-10 medium-dose and high-dose groups were weakened （P＜0.01）. CONCLUSIONS： The viscosity-average relative molecular mass and deacetylation degree of CTS-10 in CTS degradation derivatives are lower with stronger antioxidant activity， and show certain preventive effects on drug-induced liver injury fibosis in mice.