Polysaccharide yield was as index, and optimal ultrasonic-assisted extraction process of polysaccharides from sporophyll of Undaria pinnatifida was investigated by Box-Behnken design based on single factor tests. Meanwhile, the antioxidant activity of the polysaccharide was also studied by electron spin resonance (ESR) technology. It turned out that the optimal extraction conditions were as follow: ratio of water to material was 90︰1 (V∶m), ultrasonic time was 6.5 min and ultrasonic power was 440 W, extraction time was 2 h, respectively. Polysaccharide yield achieved 7.71% under that condition. And the polysaccharide from sporophyll of Undaria pinnatifida gave potent scavenging activity against DPPH· and ·OH, the IC₅₀ were 0.24 mg/mL and 0.28 mg/mL, respectively.