Abstract: Cordyceps militaris was used as the test material, on the base of single factor experiment, the response surface design experiments were used to optimize the liquid fermentation conditions of Cordyceps militaris after adding antioxidants while the content of cordycepin was taken as the evaluation index.And the activities of glutathione peroxidase（GSH-Px）,superoxide dismutase（SOD） and catalase（CAT） in the fermentation system were also measured to investigate the effect of antioxidants on the metabolism of the fermentation system, in order to improve cordycepin yield in liquid fermentation of Cordyceps militaris,and to explore the relationship between the addition of antioxidant substances and the activity of pivotal oxidoreductase and mycelial morphology.The results showed that Na₂SO₃ was selected as the antioxidant additive, and the fermentation time was set as 28 days.The response surface experiments showed that the optimal fermentation conditions were Na₂SO₃ addition of 10.2 mg·mL^-1,fermentation temperature of 24.3 ℃,and initial pH of 7.78.The content of extracellular cordycepin was 1 188.165 μg·mL^-1,which increased by 55.16% compared with blank group.The results of enzyme activity assay demonstrated that the addition of Na₂SO₃ significantly enhanced the activities of SOD and GSH-Px, especially that of GSH-Px increased by 47.51%.Furthermore, scanning electron microscopy showed that the morphology of the mycelium changed obviously, and the interior structure changed from porous to dense under the optimal conditions.The results suggested that the addition of antioxidant could regulate the oxidative stress in the fermentation system and then affect the metabolism of cordycepin.