The water chestnut as raw material, the extraction conditions of polysaccharide were studied by single factor and orthogonal experiment, antioxidant activity on polysaccharide from water chestnut was detected, respectively. The results showed that with distilled water as extraction solvent, extracting process of polysaccharide from water chestnut had microwave power of 600 W, solid-liquid ratio of 115, extracting time of 3 min, under these conditions, the extraction rate of polysaccharide could reach 8.12%, and the polysaccharide purity could up to 83.2%. The polysaccharide from water chestnut had strong reduction power and scavenging ability to hydroxyl radicals in the range of 0.01～0.10 mg/mL mass concentration, and the removal rate of hydroxyl radical reached 46.18% when the concentration of polysaccharide was 0.10 mg/mL.