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<i>Ulva</i> polysaccharides present several physiological activities including antiviral, antitumor and anti-plasmodial effects. However, current processing usually results in low yields and high prices, thus lacking commercialization potential. The aim of this study was to develop an efficient method for the extraction of <i>Ulva</i> polysaccharides with high biological activity. The effect of cell wall-degrading enzymes including cellulase, hemicellulase, pectinase and protease on <i>Ulva</i> polysaccharide extraction was studied by statistical mixing design. Using the most effective enzyme preparations as the basic components, the optimal proportions of the enzyme mixture were determined as follows: cellulase 35.3%, pectinase 34.5%, alkaline protease 30.2%, which increased the polysaccharide yield from 6.43% in the absence of enzymes to 26.68%. Subsequently, through response surface analysis, the optimal conditions were determined: enzyme concentration of 1.5%, enzymatic time of 1.1 h, ultrasonic time of 90 min and enzymatic temperature of 60 °C. Under the optimal extraction conditions, the extraction yield of <i>Ulva</i> polysaccharides could be increased to 30.14%. Moreover, extracted polysaccharides exhibit strong antioxidant properties in DPPH, ABTS, hydroxyl radical, superoxide radical and H₂O₂-induced cellular damage models. This study laid a solid foundation for the use and development of <i>Ulva</i> polysaccharides.