Crude polysaccharides obtained from Prunella vulgaris Linn by boiling water extraction and ethanol deposition, was decolorized and deproteinized with HZ-820-type resin adsorptive and further purifed by DEAE-Sepharose Fast Flow ion-exchange column chromatography and Sephadex G-50 Gel column chromatography. After that, the physicochemical properties of purified acidic polysaccharide were analyzed by gas chromatography, infrared, thermal analysis and scanning electron microscopy. Finally, antioxidant activities of it were also studied. The results exhibited that the acidic polysaccharide consisted of rhamnose, xylose, arabinose, glucose and mannose, the content of them were 0.9857％、 1.7568％、3.2029％、1.9753％ and 0.6806％, respectively. The weight-average molecular weight of acidic polysaccharide was 57234Da. The acidic polysaccharide began to degrade at 230℃ and the degradation ended at 600℃, which have better thermal stability. Meaning while, it have smooth surface, random coil and present thin slice. The antioxidant activity showed that acidic polysaccharide of Prunella vulgaris Linn had eliminating capability on NO2- radical, and also had significant reducing power and the chelating ability on ferrous ions.