Abstract: In order to prepare low-molecular-weight polysaccharides with high antioxidant activity, low-molecular-weight Enteromorpha polysaccharides were prepared by the method of microwave-assisted H₂O₂/V_C degradation using Entermorpha prolifera from the coastal area of Yingkou, Liaoning Province as raw materials. In the present thesis, DEAE-52 and Sephadex G-100 were used for chromatography separation and physicochemical properties and antioxidant activity of Enteromorpha polysaccharide were compared before and after degradation. Then structural characterization of the degraded Enteromorpha polysaccharide was obtained by infrared spectroscopy, and monosaccharide composition of LEPⅢa (obtained by separation) was analyzed by thin layer chromatography (TLC) and high performance liquid chromatography (HPLC). Results showed that low-molecular-weight Entermorpha polysaccharide could be prepared in a short time (15 min) by microwave-assisted degradation with H₂O₂/V_C and three kinds of low-molecular-weight Entermorpha polysaccharide LEPⅠ, LEPⅡ and LEPⅢa with M_w being (23.6±0.5), (24.6±0.6) and (22.9±0.5) kDa respectively were obtained by chromatographic separation. Compared with untreated Enteromorpha polysaccharide, low-molecular-weight polysaccharide obtained by microwave-assisted H₂O₂/V_C degradation were significantly reduced (P<0.05). The antioxidant activity was significantly enhanced (P<0.05). Molecular weight of the degradation products was lower than 30 kDa, with viscosity lower than 2 mPa·s. The total antioxidant capacity of LEPⅢa with a concentration of 8 mg/mL increased from (315.2±10.3) U/mL to (661.8±18.3) U/mL, and reducing capacity of ferric ion increased from 51.3%±1.0% to 83.4%±2.2% after degradation. Infrared spectroscopy of LEPⅢa showed that LEPⅢa was a kind of sulphated polysaccharide with sulfate accounting for 14.6%±0.5%. TLC and HPLC analysis showed that LEPⅢa was a low-molecular-weight polysaccharide containing sulfuric acid group, composed of glucose, rhamnose, uronic acid, etc., with molar ratio of D-(+)-mannose, L-(+)-rhamnose, D-glucuronic acid and D-(+)-glucose being 0.97:4.43:3.13:24.8. Thus, the present thesis concluded that microwave-assisted degradation with H₂O₂/V_C used to prepare low-molecular-weight Enteromorpha polysaccharides could significantly improve physicochemical properties and antioxidant capacity of polysaccharide, and the prepared Enteromorpha polysaccharides could be used as a functional component in antioxidant-related functional foods and health care products.