Abstract: Two lightly yellow sponge polysaccharides SP1 and SP2 were extracted from Sargassum cinereum, after ultrasonic extracting in NaOH solution, deprotein through Sevage method, column separation, dialysis and lyophilization. Maximum yield of polysaccharide was obtained when the algae was ultrasonic extracted in 2 w% NaOH at 80℃ for 1 h and continuously 4 h in water bath, being 13.4%. The result of phenol-sulfuric colorimetric reaction was positive. UV and electrophoresis measurements showed that SP1 and SP2 have high purity. According to GPC, the molecular weight of SP1 and SP2 is about 36kDa and 77kDa, respectively. SP1 and SP2 both have characteristic FTIR absorbances of sulfate, carboxyl group, and α-D-galactopyranosides. SP1 and SP2 showed good antioxidant activity. The hydroxyl radical scavenging capacity of SP1 and SP2 aqueous solution (10 mg/mL) was 87.2% and 61.5%, respectively. SP1 and SP2 also showed good ability to inhibit HL-60 cell proliferation. The anti-cancer activity of SP2 was higher than that of SP1.