Ultrasonic-assisted extraction of polysaccharides from Gelidium pacifium Okam. was optimized by response surface methodology based on the single factor experiment, The experiment results showed that the optimal extraction conditions were ultrasonic temperature 85℃, extraction time 50 min, and material-to-liquid ratio of 36 mL/g. Under these extraction conditions, the extraction yield of polysaccharides was 31.21%. The antioxidant activities of the extracted polysaccharides were evaluated by measuring the scavenging ratio of 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical,2,2'-zainobis (3-ethylbenzothi azoline-6-sulfonic acid) ammonium salt radical (ABTS⁺) and superoxide anion radical. The results of antioxidation experiment indicated that the polysaccharides from Gelidium pacifium Okam. removed DPPH radical, ABTS radical and superoxide anion radical, showing a dose-effect relationship. The antioxidant activity of polysaccharides obtained by ultrasonic-assisted extraction was higher than that obtained by aqueous extraction.