Objective To investigate the separation of extracellular polysaccharide from Porphyridium cruentum and its anti-HBV activity in vitro.Methods The extracellular polysaccharide(EP) of Porphyridium cruentum was extracted and isolated from the culture medium.HBeAg was assayed by ELISA and MTT methods.Results Element analysis showed that the extracellular polysaccharide contained N 0.82%,C 32.91%,H 6.19% and amino acid(2.49%).The IR and UV spectrum of EP determination showed typical absorption curve of polysaccharide,pyranglycoside linkage,sulfate ester.EP had a significant inhibition to HBeAg excretion with low toxicity on the cells and a better therapeutic index(TI).Conclusion The results suggested that EP has inhibitory effect on the hepatitis B virus in vitro.