Ginseng polysaccharide was extracted from ginseng under ultrasound, with ionic liquid as extraction solvent. The technical conditions were investigated based on single factor and orthogonal tests. The total sugar content in ginseng polysaccharide was determined by phenol sulfuric acid method. Meanwhile, compared with ascorbic acid, the antioxidant activity of ginseng polysaccharide was evaluated by scavenging DPPH free radical, hydroxyl free radical and ABTS free radical. The optimal extraction conditions of ginseng polysaccharide were shown as follows: the mass concentration of 1-hexyl-3- methylimidazolium bromide solution was 8 g/L which was used as extraction solvent, material-to-liquid ratio (ratio of ginseng to aqueous solution of ionic liquid)of 1:40 (g：mL), extratction time of about 30 min and extraction temperature at 80 ℃. The extraction amount of ginseng polysaccharide reached 172.89 mg/g. The total sugar content (mass fraction) in ginseng polysaccharide was 65.9%. The monosaccharide composition of ginseng polysaccharide was determined by HPLC after further purification，the monosaccharide groups of ginseng polysaccharide are: mannose, ribose, galacturonic acid, glucose, galactose, arabinose and fucose, the mass ratio of them was 4.8∶6.43∶10.13∶15.71∶0.5∶4.3∶1.1. The ginseng polysaccharide exhibited a certain ability of scavenging DPPH free radical, hydroxyl free radical and ABTS free radical. The antioxidant activity increased with the increase of the concentration of ginseng polysaccharide, which was inferior to ascorbic acid.