The crude polysaccharide from Amaranthus caudatus L. was extracted by water extraction and alcohol sedimentation method then purified by AB-8 resin. The extraction and purification process was optimized by single factor experiment and orthogonal experiment. Then the antioxidant properties in vitro was measured using hydroxyl radical scavenging activity, superoxide radical scavenging activity and DPPH radical scavenging activity compared with vitamin C. The result showed that optimized extraction condition was ethanol concentration 80%, ratio of solid to liquid (g/mL) 1∶20, extraction temperature 95℃, extraction time 6h. While optimized purification condition was adsorbent concentration 1.00mg/mL, pH value of adsorbent 5, adsorption flow speed 2BV/h, ethanol concentration 25%, and elution flow speed 2BV/h. Under these optimized conditions, the yield of crude polysaccharide was (1.421±0.081)mg/g and the recovery ratio of purification was 44.99%±1.23%. Also the antioxidant detection experiment showed that the crude polysaccharide from Amaranthus caudatus L.performed good antioxidant properties in vitro.