Abstract: The ocean is a treasure trove of natural polymers with diverse chemical structures and biological activities, among which polysaccharides represent one of the most abundant marine resources. Fucoidan, derived from brown algae, possesses significant biological activities, including antioxidant, anti-tumor, immune-boosting, liver and kidney function-improving, and blood sugar-regulating effects. It has unique application values in food, medicine, and materials. Compared with natural fucoidan, low molecular weight fucoidan has higher water solubility and bioavailability, and stronger prebiotic activity, which has attracted much attention. This paper reviews the structure, extraction, purification, and degradation methods for the preparation of low molecular weight fucoidan, and summarizes and prospects the biological activity of low molecular weight fucoidan , which provides a theoretical basis for further study and a reference for the high-value development and utilization of marine algae resources.