Abstract: To solve problems of the low yields and dark color of Sargassum horneri polysaccharides which were produced according to the traditional extraction method under the high-temperature and alkalic condition, this study attempted to produce S. horneri polysaccharides at a lower alkaline treatment temperature of 60 ℃ and optimize the process parameters by the response surface methodology. The combined scores of alginate and fucoidan yields and whiteness index (WI) were used as assessment indices to identify the optimal process condition. Under the optimal extraction condition, the concentration of NaClO addition was changed to investigate its effect on various characteristics of the produced alginate and fucoidan. The results showed that the polysaccharide comprehensive score was significantly correlated with the alkaline concentration, the extraction time, and the dosage of NaClO (P<0.01). The optimum reaction condition was extraction time of 7.0 h, alkaline concentration of 3.7% (w/v), and NaClO dosage of 2.7 mL (4%, w/v). Under this condition, the fucoidan yield was 9.90%, the alginate yield was 19.95%, the fucoidan WI was 62.10%, the alginate WI was 87.83%, and the comprehensive score was 44.84. The effect of a bleaching process on polysaccharide properties showed that the addition of NaClO decreased the molecular weight, viscosity, and M/G ratio of alginate, while the gel strength was increased with the moderate NaClO addition. Meanwhile, the purity of fucoidan was improved, the content of sulphate groups and the antioxidant activity were significantly increased (P<0.05). In this study, the preparation process of two polysaccharides from S. horneri was systematically investigated to provide theoretical guidance and practical experience for the industrial utilization of S. horneri resources.