The invention discloses a preparation method of a blended compound fertilizer, and belongs to the technical field of compound fertilizer production. The blended compound fertilizer is prepared from, by weight, 1200-1600 parts of fermentation mother solution dry powder, 200-300 parts of diammonium phosphate, 200-400 parts of potassium chloride, 5-10 parts of magnesium sulfate, 5-12 parts of ferrous sulfate, 1-5 parts of boric acid and 1-5 parts of zinc sulfate. The preparation method comprises the following steps that the raw materials are proportionally mixed to be uniform, then, granulation, drying and screening packaging are performed, and the finished blended compound fertilizer is obtained. According to the preparation method of the blended compound fertilizer, the problem that environmental pollution and energy waste are caused due to a spraying granulation technology is solved, the drying temperature is 60-160 DEG C, the dissipated energy is greatly lowered, meanwhile, organic matter in the raw materials is not likely to be destructed, the content of free amino acid is high, and the organic matter can be sufficiently absorbed and utilized by crops; due to the fact that the compound fertilizer has a large number of abundant microelements, the compound fertilizer can effectively improve soil, promote crop growth and improve the agricultural product quality.