Abstract: Objective: A 2,2-azobis(2-methylpropylimidazole) dihydrochloride(AAPH)-stimulated zebrafish model was studied and used for investigating the antioxidant activities of a fucoidan(SFPS) isolated from Sargassum fusiform(S. fusiform). Methods: SFPS extracted from S. fusiform and the chemical composition of SFPS were determined. The scavenging capacities of SFPS on 1-diphenyl-2-picrylhydrazyl(DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt(ABTS) radicals were determined to evaluate its antioxidant activity in vitro. Then the AAPH-induced oxidative stress model of zebrafish was studied and optimized. Three phenotypic indexes(survival rate, yolk sac size and heartbeat rate) as well as levels of cell death and reactive oxygen species(ROS) were used to evaluate the antioxidant activity of SFPS in vivo. Results: The SFPS contained 75.30%±1.77% carbohydrates content, 21.39%±1.07% sulfate content, 1.78%±0.19% protein content and 1.47%±0.02% phenolic content. Compared with the crude polysaccharide(SFPSC), lower IC₅₀ values of SFPS for DPPH and ABTS were determined to be 0.59 and 0.69 mg/mL, respectively. Besides, SFPS was nontoxic to zebrafish embryos in the range of 50~200 μg/mL and dose-dependently protect zebrafishfrom AAPH induced oxidative damage. At the optimal concentration of 200 μg/mL, the survival rate of zebrafish embryos was significantly increased(100%, P<0.05), the heartbeat rate and yolk sac size were significantly reduced(101.37% and 111.80%, respectively, P<0.05), and the inhibition rate of AAPH-induced cell death and ROS production in zebrafish were up to 70.55% and 50.68%, respectively. Conclusion: SFPS would have strong antioxidant activity in vitro, and had superior antioxidant ability and oxidative damage repair ability in vivo, thus indicated that SFPS as a natural antioxidant would have a wide application prospect in the field of health food and cosmetics.