Abstract: Fucoidan, a class of bioactive fucose-rich sulfated polysaccharides, which are currently primarily extracted from brown algae, has important applications in the food and biomedical fields. Hainan Province is abundant in Sargassum oligocystum resources. We optimized the extraction of fucoidan from S. oligocystum using ultrasound-assisted hot water extraction combined with an orthogonal experimental design, as well as analyzed its acetylcholinesterase inhibitory activity and antioxidant capacity. The results show that under the conditions of a solid-to-liquid ratio of 1:50 (g·mL^-1), extraction at 75 °C for 4 h, the yield of fucoidan reached (4.94±0.06)%, which was a 398.99% increase compared to that before optimization. The extraction temperature, solid-to-liquid ratio, and extraction time had the most significant impact on the yield. The extracted fucoidan exhibited significant concentration-dependent inhibitory activity against acetylcholinesterase, with an inhibition rate of 12.10% at a concentration of 300 μg·mL⁻¹. Furthermore, the fucoidan showed potent scavenging effects on DPPH, hydroxyl, and superoxide anion radicals, with scavenging rates of 11.25%, 41.93% and 44.88%, respectively, at a concentration of 5 mg·mL⁻¹. The results indicate that fucoidan extracted from S. oligocystum exhibited significant acetylcholinesterase inhibitory activity and antioxidant capacity, demonstrating potential for development into functional foods or pharmaceutical agents related to Alzheimer's disease.