The invention relates to a biodegradable plastic and a preparation method thereof, belonging to the technical field of plastic processing, wherein the parts by weight of each component are 16-21 parts of chitin, 2-5 parts of sodium fluorosilicate, 2-5 parts of gelatin, 1~3 parts of calcium carbonate, 1~3 parts of glycerin, 18~22 parts of superfine starch, 1~3 parts of glycerin, 0.4~0.7 parts of stearic acid, 0.4~0.7 parts of oleic acid, 15 parts of low density polyethylene ~20 parts, 15~20 parts of high-density polyethylene, 0.4~0.7 parts of manganese acetylacetonate, the preparation method is to raise the temperature of the container to 75~85°C, put the above raw materials in the container in proportion High to 125~130°C, keep stirring at this temperature and react for 1.5~1.8h, adjust the pH of the reaction solution to 5.8~6.5 with hydrochloric acid to obtain a uniformly dispersed solution; pour the solution into a preheated mold for foaming, The biodegradable plastic is obtained by thermal treatment and demoulding; compared with the same type of products on the market, the plastic has the advantages of lower production price, faster degradation speed, non-toxic, and no pollution to the environment.