The extraction process of the red pigment from Amaranthus spinosus L was optimized and its antioxidant activities were investigated in in vitro tests. The optimal extraction conditions were obtained by the single factor analysis using the absorbency as an index and its reducing power, and scavenging effect of hydroxyl radical and DPPH radical were studied.The results showed that the optimum conditions were as followed:extraction solvent 20% ethanol, the solid-liquid ratio 1∶30, temperature 40℃, extraction times 2, and extraction time 1h. Reducing power of red pigment was 1/4 of VC, the half-inhibitory concentration (IC₅₀) of hydroxyl radicals and DPPH· were 0.44 and 0.14mg/mL. The red pigment from Amaranthus spinosus L. has antioxidant activities, which were dose-depended.