The anthocyanins were extracted from purple sweet potato (PSP) tuber. Influencing factors of ultrasonic-enzyme combined extraction of anthocyanins were studied by single factor experiments. On the basis of this, the extraction process was optimized by three factors and three levels of response surface analysis method, using yield rate as response value. The optimal process parameters of the extraction process were: extraction temperature 60 ℃, extraction time 35 min, liquid to solid ratio 361（mL/g）. Under these conditions, the yield of anthocyanins from purple sweet potato was 2.003 mg/g. The results showed that the determination of reducing power of purple sweet potato anthocyanins and the scavenging rate of ·OH increased with the increase of concentration. The scavenging rate of ·OH was 80.2％ at the concentration of 30.00 mg/L. The study revealed that the anthocyanins from purple sweet potato have high antioxidant activity in vitro and can be used as a natural antioxidant.