Cordyceps taii, a folk medicine native to south China, is a promising source for the development of healthy foods and new drugs due to its diverse pharmacological properties including antioxidative stress effect. The antioxidant properties of alkali-soluble polysaccharide fractions from C. taii were systematically investigated, such as the scavenging abilities on DPPH (DPPH•), hydroxyl (•OH) and superoxide anion (•O2-) free radicals, the reducing power and the Fe2+chelating ability. Among these assays, the refined polysaccharide fraction (ARP) presented more excellent scavenging abilities towards •OH and •O2- with EC50 values of 2.72 ± 0.80 and 10.46 ± 1.97 mg/ml compared with the crude polysaccharide fraction (ACP). While the ACP showed slightly stronger scavenging activity on DPPH•, the reducing ability than the ARP. In comparison with the positive control, the ARP had more significantly scavenging •O2- activity, and had a moderate reducing ability and scavenging other free radical activities in a dose-dependent manner. A pure polysaccharide fraction from the ARP was found to be mainly composed of glucose, galactose and mannose (molar ratios=1.14: 1.00: 1.66) with series α- (1, 4) glucosidic bond, and the mannose were assigned as the backbone of the polysaccharide structure accordingly. Taken together, the alkali-soluble polysaccharide is an important antioxidant component in C. taii, and exerts the antioxidant properties by scavenging free radical.