Abstract: Polysaccharide is one of the globally recognized micro-molecules used in biomedicine, food preparation and production, which has the bioactivities of anti-tumor, immunomodulatory, hypoglycemic, antioxidant and anti-bacteria. Recently, more and more researchers have studied the bioactivities and applications of polysaccharides derived from animals, microorganisms and plants. In general, bioactivities of polysaccharides are affected by the monosaccharide composition, molecular weight, glycosidic type, branching degree and spatial conformation in which. However, how to realize the design, construction, screening and efficient production of bioactive polysaccharides has always been the focus and difficulty of polysaccharide research. Therefore, based on the relationship between molecular weight and bioactivities of polysaccharide, the effects of molecular weight on antioxidant, anti-tumor, anti-inflammatory, probiotic, immunomodulatory and bacteriostatic activities of polysaccharides were introduced. Furthermore, this review discussed and prospected the variation of polysaccharide bioactivities induced by its molecular weight differences. In conclusion, we hope that this work could provide some theoretical references for the future research and application of bioactive polysaccharides from the perspective of molecular weight.