Polysaccharides were extracted from the roots of Arctium lappa L. (ALPs) using response surface methodology with ultrasonication. A central composition design was used to optimize extraction parameters by maximizing the polysaccharide extraction yield. The modified optimal conditions were as follows: water to raw material ratio of 31 mL/g, ultrasonic power of 158 W, extraction time of 83 min, and extraction temperature of 50 °C. Furthermore, fractions of ALP40-1, ALP60-1, and ALP80-1 were obtained for chemical and antioxidant activity analyses after purification. Results indicated that the three fractions had a molecular weight of 218, 178, and 60 kDa, respectively, and were composed of mannose, glucose, fructose, and galactose. ALP60-1 exhibited strong scavenging activities on 1,1‑diphenyl‑2‑picryhydrazyl, hydroxyl, and superoxide radicals. These results demonstrate that ultrasonic-assisted extraction is a very effective method for extracting ALPs, and ALP60-1 is a potential novel natural antioxidant. However, further structure elucidation and in vivo experiments are required.