The process of extracting the insoluble dietary fiber (IDF) of purple potato by ultrasonic-assisted enzymatic method was studied, and its antioxidation in vitro was also evaluated. The effect of five factors (ratio of material to liquid, ultrasonic time, ultrasonic power, alpha amylase dosage and enzyme hydrolysis temperature) on insoluble dietary fiber extraction rate were determined. Orthogonal optimization test was carried out on the basis of single factor test, and the antioxidant level of IDF was evaluated by hydroxyl radical (·OH) in vitro. The single factor test showed that among the factors, the enzymolysis temperature played the most significant role on the extraction. Further orthogonal experiment get the optimum process parameters of purple potato IDF as follows: the ratio of material to liquid 115 (g/mL), ultrasonic time 40 min, ultrasonic power 160 W, and the enzymolyzing at 75 ℃. Under this condition, the IDF extraction could be as high as 38.32%. Finally, the anti-oxidation of purple potato IDF was determined by the method of removing hydroxyl radical (·OH). The results showed that the purple potato IDF has good cleaning effect on ·OH, and a certain concentration-response relationship was observed. Its IC₅₀ was calculated to be 39.27 mg/mL, and indicated that the antioxidant activity was stronger than V_C.