Abstract: To improve the extraction yield and the antioxidant activity, the Lentinan polysaccharides were obtained by ultrasonic assisted water extraction in this study.And a Box-Behnken design was employed to optimize the ultrasound extraction based on single factor experiments.The polysaccharides extracted under the optimized condition were conducted to determinate the antioxidant activity in vitro and their structures.The optimal conditions were as follows: time 50min, temperature 62℃, ultrasound power 640W。Under optimal conditions, the extraction yield achieved 7.34%, and the Oxygen Radical Absorbance Capacity (ORAC value) was 821.35μmol Trolox /g, with no significant difference to the predictive values.The scavenging activity of ABST radicals, hydroxyl radicals and reducing power were employed to evaluate the antioxidant of polysaccharides.The results showed that the extraction polysaccharides possessed considerable higher antioxidant activity.The polysaccharides showed typical absorption peaks by infrared spectroscopy, indicating that the chemical structure did not change significantly compared with those of polysaccharides prepared by traditional hot water extraction.