Abstract: Chitin is a major component of shrimp and crab shells in fishery production.There are about 6 million to 8 million tons of discarded crab and shrimp shells produced each year.These discarded shrimp shells are usually dumped in landfills or discharged into the ocean,causing serious environmental pollution and waste of resources.Useful chemicals such as protein,calcium carbonate and chitin contained in shrimp and crab shells are ignored.Therefore,finding sustainable methods to make rational use of the crustacean shell has attracted widespread attention.The traditional method for obtaining chitin,chitosan and chitooligosaccharides is through the strong acid chemical digestion method.With the improvement of people's environmental awareness,this method has been difficult to sustain.In addition,chitosan and chitosan oligosaccharides produced by chemical methods are difficult to control their degree of polymerization,degree of acetylation and acetylation mode,so it is difficult to study the relationship between the structure and function of chitooligosaccharides.Chitin deacetylase(E.C.3.5.1.41)is an enzyme that can convert chitin oligosaccharides into chitosan oligosaccharides under mild conditions.Chitosan oligosaccharides with specific acetylation patterns have become an important research direction.Chitosan,a chitin deacetylated product,is a linear polymer composed of β-1,4-N-glucosamine,which is the only basic polysaccharide in nature.The partially acetylated chitooligosaccharide(COS)consists of N-acetylglucosamine(GlcNAc,A)and glucosamine(GlcN,D)and is a complex biopolymer with a variety of biological activities and potential applications.Chitosan has a broad spectrum of antibacterial properties,water retention,reproducibility,good biocompatibility,degradability,cell membrane permeability,high drug loading,non-toxic side effects,etc.Through glutaraldehyde after cross-linking,grafting,acetylation and other chemical and physical modification,it is widely used in medicine,environmental protection,food,agriculture,paper,cosmetics and other fields.Partially acetylated chitooligosaccharides are potent biological agents with anti-inflammatory,anti-tumor,immunomodulatory,neuroprotective and other important pharmaceutical activities.Their biological activities are thought to depend on their structure,ie their degree of polymerization and degree of acetylation,and their mode of acetylation.Chitin deacetylase(CDA)can directly deacetylate from chitin oligosaccharides to form chitosan oligosaccharides,which are widely present in eukaryotes and prokaryotes.The reaction conditions for the bio-enzymatic production of chitosan are mild,non-polluting,and the degree of polymerization,deacetylation and deacetylation of the product chitosan oligosaccharides are single,so chitin deacetylase has attracted wide attention.It has been reported that chitin deacetylase was successfully constructed into Pichia pastoris or Escherichia coli for heterologous expression.Chitin deacetylase(CDA)can directly deacetylate from chitosan oligosaccharides to form chitosan oligosaccharides,which are widely present in eukaryotes and prokaryotes.Several chitin deacetylases have been expressed in prokaryotic and eukaryotic hosts.The reaction conditions for the bio-enzymatic production of chitosan are mild,non-polluting,and the structure of chitosan oligosaccharides are single,so chitin deacetylase has attracted wide attention.In view of the importance of this enzyme for industrial development,this paper reviews the sources of chitin deacetylase,deacetylation mode,catalytic mechanism and application of chitosan oligosaccharides.