The extraction of crude polysaccharides from Semen euryales was studied and the scavenging activity of hydroxyl radical of the polysaccharides was also discussed.The effects of extraction temperature,extraction time,ethanol concentration,liquid to solid ratio on the extraction yield of the crude polysaccharides and the scavenging activity of hydroxyl radical of the polysaccharides were investigated.Single factor experiments and the orthogonal experiments was designed by Design-Expert software.The model was established by the response surface methodology analysis and the optimal extraction condition was obtained as follows: extraction temperature 92℃,extraction time 6h,ethanol concentration 85%,liquid to solid ratio 26:1,the extraction yield of crude polysaccharides was 0.63mg /g and the scavenging activity of hydroxyl radical was 46.42%.