An effective method to extract Cyanobacteria polysaccha-ride was established in this study and a high yield of polysaccharide (25.1%) was achieved by optimizing extraction condition (hot water with ultrasonic treatment). After purification, a main purified polysaccharide (CB-2-1) with molecular weight of 8.4 kDa was collected and the compositions included rhamnose, arabinose, xylose, mannose, glucose and galactose, with mole percentages of 0.51%, 13.35%, 18.21%, 51.38%, 4.87% and 11.67%, respectively. The results of NMR showed that the CB-2-1 component of cyanobacteria polysaccharide was mainly α-type glycosidic bond. Moreover, the purified CB-2-1 polysaccharide had great anti-oxidant activities. The highest scavenging activities of DPPH and hydroxyl radicals with 2 mg/mL CB-2-1 were 98.75% and 72.64%。