Response surface methodology (RSM) was used to optimize experimental conditions for ultrasonic-assisted extraction (UAE) of antioxidant crude polysaccharides (CPS) from Trapa quadrispinosa stems. Antioxidant capacity of polysaccharides was determined by Ferric-Reducing Antioxidant Capacity (FRAC) assay with the method of 1, 10-phenanthroline. The maximum yield of polysaccharides (2.78±0.16%) was obtained under optimal extraction conditions of extraction time, 41min; ratio of water to material, 31.5mL/g; and extraction temperature, 58°C. The maximum antioxidant capacity (19.02±0.24μmol Fe2+/g) was obtained under the optimal extraction conditions of extraction time, 38min; ratio of water to material, 32mL/g; and extraction temperature, 56°C. These two values were agreed well with predicted yield (2.75%) and antioxidant capacity (18.77μmol Fe2+/g). Antioxidant activities of CPS were investigated by various assays. The results showed that CPSUAE obtained by UAE showed higher 1,1-Diphenyl-2-picrylhydrazxyl (DPPH), 2,2-azinobis(3-ethylbenzthiazoline)-6-sulfonic acid (ABTS) radical scavenging activities and stronger reducing power, total antioxidant capacity compared with CPSHWE obtained by hot water extraction (HWE).The results indicated that UAE is an advisable method for extraction of antioxidant polysaccharides from the stem of T. quadrispinosa and polysaccharides could be explored as potential antioxidant to use in medicine or functional food.