Background: Sargassum henslowianum has become an important source for the food industry as well as medicinal applications. In recent years, varieties of bioactive components in S. henslowianum and its activities have reported. However, the optimized extraction conditions of polysaccharides and polyphenols in S. henslowianum was unknown, and their activities need to be explored more. Objective: This study was to optimize the ultrasound-assisted extraction of polysaccharides and polyphenols from S. henslowianum and test the biological activity of the extract. Materials and Methods: Response surface methodology was used to optimize extraction conditions, and the antioxidant activity of the extracts was evaluated by radical scavenging assay, α-Glucosidase inhibition, and cytotoxicity on MCF-7. Results: The optimal conditions of extracting polysaccharides were shown as following: ultrasonic time for 40 min, ultrasonic power for 330 W, solid-to-liquid ratio for 1:36 g/mL, the extraction yield reached 12.63% under above parameters. The optimum conditions of ultrasonic-assisted extraction of total polyphenols were as following: ultrasonic time for 102 min, ultrasonic power for 377 W, alcohol concentration for 62%, under these conditions, the extraction yield reached 11.45%. Besides, the extracts of polyphenols possessed stronger activity. Conclusion: This study provides the scientific guidance for further exploitation and utilization of S. henslowianum.