Abstract: Changes in the composition of organic acids and in vitro antioxidant activity of Citrus medica Jiaosu were investigated. During fermentation,scavenging rate on superoxide radical,hydroxyl radical and ABTS radical showed an increasing trend generally. DPPH radical scavenging rate and reducing power initially increased,then decreased,and gradually increased afterwards. The main organic acids of Citrus medica Jiaosu were L-malic acid and citric acid during 5~15 d fermentation,while they were lactic acid,acetic acid and L-tartaric acid during 40~90 d. Meanwhile,shikimic acid,ascorbic acid,succinic acid and oxalic acid were found in Citrus medica Jiaosu. The amount of total organic acid increased firstly,then decreased and then increased again until reaching its peak of(9.600±0.18) mg/mL at 90 d,growing by 105.47% compared with the amount in Citrus medica Jiaosu at 5 d. The different time points and species of organic acids were classified into two and three groups respectively by cluster analysis when rescaled distance cluster was 15. Both superoxide radical and hydroxyl radical scavenging rate showed extremly(P<0.01)significant positive correlation with the contents of lactic acid(the spatial correlation coefficient was 0.885 and 0.680 respectively). The research results revealed the changes in the composition of organic acids and in vitro antioxidant activity of Citrus medica Jiaosu,and provided a theoretical reference for precision preparation of Citrus medica Jiaosu.