The in vitro antioxidant activities of polysaccharides from Allium macrostemon Bunge were evaluated by radical scavenging activity, and three kinds of polysaccharides were extracted from fractional alcohol precipitation. The preliminary structures and compositions were analyzed by infrared spectroscopy. During in vitro antioxidant assay, AMP40 exhibited the strongest scavenging activity than the others in scavenging hydroxyl radical (?OH) and superoxide radical (O2-?). The in vivo antioxidant activities of crude AMP were investigated that against CCl4-induced acute liver damage in Kunming mice. The results showed that pretreatment with AMP40 could markedly improve the activities of those antioxidant enzymes in CCl4-treated mice. The antioxidative system tended to be normalized by the protective action of AMP40, and scavenging activity of AMP40 was in a dose-dependent manner. It also showed that antioxidant properties directly related to the molecular weight and the uronic acid content of polysaccharide.