The invention relates to a method for preparing chitin and chitosan by coupling shrimp and crab shells through a hydrothermal/two-step enzymic method. The method comprises the following steps: removing impurities from shrimp and crab shells, and crushing the shrimp and crab shells; adding organic acid for decalcification treatment, placing filter residues into a hydrothermal reactor, and feeding a reaction medium for hydrothermal reaction; then rapidly relieving the pressure in the reactor, collecting a hydro-thermal treatment product, carrying out solid-liquid separation, and adding protease into a solid material to carry out enzymolysis deproteinization treatment; separating the deproteinized product to obtain solid residues, drying the solid residues to obtain chitin, and adding chitin deacetylase into the chitin for enzyme treatment to prepare chitosan; and finally, treating the organic acid decalcification solution and the protein enzymatic hydrolysate to obtain organic calcium and protein peptide. According to the method, the decalcified shrimp and crab shells are modified through a hydrothermal method, the compact structure of the shrimp and crab shells is broken, the accessibility of subsequent protease and deacetylase is promoted, the enzymolysis efficiency is improved, poly-generation of organic acid calcium, protein peptide, chitin and chitosan is achieved, the production process is rapid, stable and free of pollution, and large-scale industrial application is easy to achieve.