Abstract: Objective: To study the antioxidant activity of Sipunculus nudus Linnaeus from the South China Sea in vitro.Methods:The homogenate of S.nudus was extracted by ethanol, ethyl acetate, and water in order to get fractions with different polarities, and then the antioxidant activity of the extracts was tested, compared with one synthetic antioxidants ascorbic acid.The chemical components of extracts with antioxidant activity were analyzed.Results: The results showed that the water fraction exhibited high antioxidant activity, the ethyl acetate fraction and ethanol extract exhibited low antioxidant activity.59 compounds were found in the ethyl acetate fraction, which was mainly composed of unsaturated fatty acids and steroids. The water fraction had many components, including protein, total sugar, taurine, SOD and so on. Conclusion: Unsaturated fatty acids, steroids, polypeptide, taurine, SOD might have contributed to the antioxidant activity of S.nudus.