The invention relates to a method for preparing a microbial feed additive. Feather meal, mannitol, glucose, sodium citrate, sodium chloride and the like are taken as raw materials, and inoculated with Bacullus subtilis YYW-1 with the number of CGMCC NO.2396 to prepare the microbial feed additive by submerged fermentation and spin flash drying technology. The active ingredients of the microbial feed additive are the Bacullus subtilis and cultures thereof, and the cultures contain metabolites such as keratinase capable of efficiently degrading feather. The product can be used in the breeding industry and the feed industry, and has the advantages of improving intestinal flora, improving digestion and absorption capacity of animals and the feed conversion and utilization rate, particularly promoting the digestion, absorption and utilization of protein materials such as the feather meal, and reducing cost.