The effects of Chinese milk vetch(Astragalus sinicus L.hereafter vetch)application rate on changes of soil labile organic carbon fractions and Ctransformation enzyme activities were studied through vetch returning sitespecific experiment.The relationships between the contents of soil labile organic carbon fractions and enzyme activities were also investigated in this study.The experiment included control,application chemical fertilizer,and four vetch application rate(30,60,90 and 120 t·hm-2)treatments.The results showed that application of fertilizer or vetch both significantly increased the content of soil labile organic carbon(dissolved organic carbon,hotwater extractable organic carbon,microbial biomass carbon,particulate organic carbon)compared with control treatment.The increase in vetch application rates led to significant increase of soil labile organic carbon content.The Ctransformation enzyme activities showed similar trend with soil labile organic carbon.In comparison with control treatment,fertilizer and vetch application increased the acitvities of cellulose,invertase,βglucosidase,phenol oxidase and peroxidase by 14% and 24%～55%,27% and 45%～187%,42% and 51%～165%,11% and 11%～24%,16% and 16%～27%,respectively.Application of chemical fertilizer treatment had similar the relative enzyme activity(the ratio of enzyme activity to microbial biomass carbon)compared with control treatment.The increase in vetch application rates significant increased the relative activities of invertase and βglucosidase,but decreased the relative activities of phenol oxidase and peroxidase compared with control treatment.Stepwise multiple regression analysis showed that the activities of cellulose,βglucosidase and phenol oxidase played the principal role in determining contents of soil labile organic carbon.