Polysaccharides and their derivatives have opened up new industrial field because of their special biochemical functions. Their biological immune, biological inhibition, antioxidant and anticoagulant properties have begun to help people enhance their physique, the fight against aging. Molecular modification is an important way to study the structure-activity relationship of polysaccharides which should be focused on. After molecular modification, various derivatives of different types of structures and biological activities are gained. Chemical modification of polysaccharides, e.g., carboxymethylation, sulfonylation, phosphorylation, acetylation, and hydroxypropylation, cationic or other derivatives, can improve their bioavailability and applications in different biological systems.