The present invention relates to a method for preparing chitin and chitosan oligosac charide. The method comprises the steps that usual raw materials such as the crust of shrimp and crab, the insect crust or the fungal mycelia, etc. are micronized through the dry process or wet process; the carapace material of the obtained fine powder raw material is decalcified with the chemical process, and then is defatted and deproteinized with the method of micro-organism compound enzyme coarse enzyme liquid co-enzymolysis, and the insect and fungus fine powder thereof is directly defatted and deproteinized; a whole cell immobilizing bioreactor of a chitin deacetylase high-yield producing strain is prepared, to perform the circulatory deacetylation to the chitin and then obtain chitosan with corresponding degree of deacetylatoion; obligate anaerobic acid-producing bacterium and high-yield producing chitosan bacterium are utilized, the chitosan is submerged and fermented in the liquid, to obtain chitosan oligosaccharide with high water solubility. The present invention has the advantages that the method is helpful to fully utilize the resources, and makes the waste to the worth, at the same time, the default of the manufacturing process of the chemical process can be avoided, the production efficiency is improved, the energy is saved, the consumption is reduced, the byproduct with corresponding high value added can be produced, the comprehensive economic benefits of the relative secondary industry are obviously improved, the industrial development is promoted, and the multi-win effect is attained.