Abstract: Polysaccharides from Hyriopsis cumingii were extracted by using methods of ultrasonic- assisted hot water lixiviation, ethanol precipitation and Sevag's deproteination.On the base of single- factor tests, parameters combination for the ultrasonic- assisted extraction of H. cumingii polysaccharides was optimized by using BoxBehnken design of response surface methodology. Antibacterial activities of H. cumingii polysaccharides were evaluated in vitro by using method of Oxford cup-inhibition zone.Results showed that optimum conditions for the extracting of H.cumingii polysaccharides were ultrasonic power 600 W, extracting temperature 50℃, extracting time48 min and ratio of water to raw material 12 mL /g. Practicing this optimal condition, extraction yield of polysaccharides from H. cumingii was 4.61%. When polysaccharide concentration was less than 40 mg /mL, no obvious inhibitory effect of H.cumingii polysaccharides on Escherichia coli and Micrococcus luteus was observed.When polysaccharide concentration was more than 10 mg /mL, polysaccharides of H. cumingii could inhibit the growth of Bacillus cereus, Staphyloccocus aureus and Bacillus subtilis in a dose- dependent manner. The antibacterial activity of H.cumingii polysaccharides against Staphyloccocus aureus was the strongest, followed by Bacillus cereus, then the inhibitory effect on Bacillus subtilis was the weakest.