The invention relates to a fertilizer for plants, and in particular relates to a controlled release composite fertilizer for soybean and a preparation method thereof. The controlled release composite fertilizer for soybean is prepared by fermented bean dregs, distillers' grains, sweet potato slices, fulvic acid, urea, diammonium phosphate, potassium chloride, zinc sulfate, fructus cnidii, sophocarpidine, radix euphorbiae lantu, salix leaves and gelatin according to a special proportion through a special preparation method. Organic fertilizer and inorganic fertilizer are combined together and are gradually released along with solubility of inorganic components and degradation of the organic fertilizer, so that release of the fertilizer is matched with growth demand of plants, thereby, further improving the yield of soybean. The fertilizer contains nitrogen, phosphorus and potash fertilizers as well as amino acids, enzymes, amides, cytokinin, beneficial biological strains and organic antibacterial substances, so that the quality of soybean is improved, and the content of trace nutritional ingredients is higher, therefore, the soybean is further in accordance with dietary standard of modern people.