Abstract: The orthogonal experiment was used to optimize the extraction process of polysaccharides from Semen Euryales by cellulase ultrasonic synergistic.The antioxidant capacity and the protecting activity of polysaccharides against DNA damage caused by UV- photolysis and H2O2 treatment were also investigated. The results indicated that the optimal conditions were as follows: liquid ratio 1 ∶ 20 ( g / m L) , p H3, cellulase 2.0%, ultrasonic time 5min, extraction time 3h, temperature 40℃. Under the optimal conditions, the extraction yield achieved 17.04%. The IC50 values of polysaccharides against DPPH· and OH· were 1.78 mg / m L and 6.96 mg / m L, respectively. However, the scavenging abilities on H2O2 and O-2· were 48.00% and 20.32% at the concentration of 10.0mg /m L. In case of protecting activity against DNA damage, 0.08 mg / m L of polysaccharides was optimum. Moreover, it promoted the DNA damage while the mass concentration of polysaccharides was above 1.0mg / m L. In conclusion, Semen Euryales polysacchariides possessed antioxidant activity and protective effects on oxidative plasmid DNA damage.