Microbial polysaccharides produced by bacteria, fungi and cyanobacteria are preserved by polysaccharides, which have attracted much attention because of their safety, non-toxicity and unique physical and chemical properties. Polysaccharides produced by microorganisms are easy to be separated from bacteria and can be industrialized through submerged fermentation. Microbial polysaccharides are used as film-forming agents and preservatives in food and pharmaceutical fields. For example, pullulan can be used for food preservation because of its tasteless, odorless, non-toxic, edible, good film-forming property and excellent oxygen barrier effect. Chitosan is a kind of nitrogen-containing polysaccharide, which is obtained by acidification from the shells of arthropods such as shrimp, crab and insects and the cell walls of lower plants such as fungi and algae. Because of its good film-forming and antibacterial effects, chitosan has been applied to the preservation of fruits and vegetables. Fresh-keeping experiments on citrus, strawberry, apple, kiwi fruit, cucumber, green pepper and tomato show that [1]: spraying 0. 1.7% ~ 2% chitosan solution can form a thin film on the surface of fruits and vegetables, which can prevent the fruits from absorbing O2 and CO2, thus delaying the ripening of fruits and achieving the purpose of fresh-keeping.