The invention relates to a biodegradable plastic and a preparation method thereof, and belongs to the technical field of plastic processing. The biodegradable plastic comprises the following raw materials in parts by weight: 16-21 parts of chitin, 2-5 parts of sodium fluosilicate, 2-5 parts of gelatin, 1-3 part(s) of calcium carbonate, 1-3 part(s) of glycerol, 18-22 parts of ultrafine starch, 1-3 part(s) of glycerin, 0.4-0.7 parts of stearic acid, 0.4-0.7 parts of oleic acid, 15-20 parts of low-density polyethylene, 15-20 parts of high-density polyethylene and 0.4-0.7 parts of acetyl acetone manganese. The preparation method comprises the following steps: heating a container to 75-85 DEG C; putting the raw materials into the container, heating the raw materials to 125-130 DEG C within 0.5-0.8 hour, and mixing and reacting for 1.5-1.8 hours at the temperature; adjusting the pH value of a reaction solution to be 5.8-6.5 by hydrochloric acid to obtain a uniformly dispersed solution; pouring the solution into a preheated mould for foaming and forming, thermalizing, and demoulding to obtain the biodegradable plastic. Compared with like products on the market, the plastic has the advantages of relatively low production price, relatively high degradation speed, no toxin materials, environmental protection and the like.