The invention relates to the technical field of chemical industry, and discloses a preparation method of a special fertilizer for kidney bean, which solves the following technical problems: the matching of the nutrient elements in the existing fertilizer does not conform to the demands of kidney bean for the nutrient elements, so that the fertilizer applied to the kidney bean can not be completely absorbed by the kidney bean, and the compounds of some elements residing in the soil change the soil structure and pollute the environment. According to the method, phosphoric acid and carbamide react to generate an intermediate, the intermediate reacts with alkaline potassium, and part of potassium dihydrogen phosphate generated by the reaction is crystallized to precipitate, so that the contents of nitrogen, phosphorus and potassium in the reaction solution are regulated to satisfy the demands for kidney bean growth. The special fertilizer for kidney bean is lower in production cost, can be largely utilized by the vegetable, and has obvious economic value.