The invention discloses a preparation method of a biodegradable coating material, relates to a preparation method of a coating material, and aims at solving the technical problem that an existing coated slow release fertilizer is relatively high in price. The method comprises the following steps: mixing corn starch with an enhancer, and then adding distilled water; adding a water-proofing agent and a plasticizer, and dropwise adding ammonium hydroxide and ethyl silicate, so as to prepare a film forming solution; carrying out curtain coating of the film forming solution on a polytetrafluoroethylene glass plate under the condition of room temperature; and drying and uncovering a film, so as to obtain the biodegradable coating material. The dosage of the prepared biodegradable coating for a starch-based slow-release fertilizer is 0.5%-1.0% of the total amount of the fertilizer, and the cost of each ton of fertilizer is increased by 72.1-144.2, and is improved by 2.87%-5.76% in comparison with that of common urea. The preparation method belongs to the field of preparation of a chemical fertilizer coating.