The invention relates to a preparation method for a dedicated soybean paste leavening agent, belongs to the field of fermentation engineering, and in particular relates to a method for preparing a leavening agent with mixed strains for soybean paste preparation. The preparation method is characterized in that two types of dedicated soybean paste solid leavening agents used in primary fermentation A and after fermentation B respectively are contained, wherein the dedicated soybean paste solid leavening agent used in the primary fermentation A comprises aspergillus oryzae, aspergillus niger, natto fungus and the like which are prepared in a pure-culture manner; the dedicated soybean paste solid leavening agent used in the after fermentation B comprises aroma-producing yeast; the spore or thallus amounts of both the dedicated soybean paste solid leavening agents are not lower than those of a 108 CFU/g bacteria agent. The method refers to the processes of soaking soybeans, cooking, wrapping the soybeans with steamed wheat flour, inoculating the leavening agent A, making koji, carrying out primary fermentation, mixing with low-salt water, inoculating the leavening agent B, carrying out after fermentation, maturing, sterilizing, checking and obtaining the finished product. The preparation method has the advantages that (1) a composite leavening agent is adopted, so as to shorten paste airing time or fermentation time during paste making, and ensure safe quality of the product; (2), the leavening agents are powdered to facilitate use; (3), the product is bright in tint, has paste flavor and ester flavor, is low in salt content, and has no foreign taste; (4), the production technology is simple.