Natural polysaccharides are synthetic macromolecules located in the cell wall and intracellular, intercellular, and secretory cells. These are essential components of life activities. Cellulose and its derivatives, chitosan, alginic acid, starch, and other natural polymer materials that exist in nature are polysaccharides. This article briefly reviews the structure and sources of several natural polysaccharides, focusing on the modification design strategies of polysaccharides such as graft polymerization, oxidation, etherification, and esterification on cellulose (mainly cellulose nanocrystals, CNC), chitosan (CTS), and other polysaccharides and derivatives in the fields of biomass energy, composite materials, wastewater treatment, and biomedicine. Based on the theory of modification and application of polysaccharides at the present stage, future research goals and modification strategy design are prospected.