The polysaccharide of Onchidium struma was extracted by using traditional hot water extraction and alcohol precipitation method，the influence of vacuum freeze drying and oven drying on the antioxidant activity of polysaccharides was investigated. The antioxidant activities of the polysaccharide was evaluated by scavenging DPPH free radical, hydroxyl free radical, surperoxide anion radical and sodium nitrite, and the total reducing capacity of polysaccharide was studied by spectrophotometric method. The results showed that the scavenging effect of freeze dried polysaccharide was stronger than the oven dried polysaccharide,the IC₅₀ for scavenging activities against DPPH free radical, hydroxyl and sodium nitrite were 4.14, 2.96 and 2.05 mg/mL,which showed the relationship to close-dependence,but the polysaccharide had little scavenging effect on surperoxide anion radical, and the reducing capacity was weak. The antioxidant activities of two polysaccharides were weaker than Vc under the same concentration.