The invention belongs to the technical field of food processing, and relates to a dietary supplement rich in soybean polypeptide and a preparation method thereof. The method comprises the following steps: A. preparing a fermentation base material, dissolving soybean polypeptide in water, adding sugar, homogenizing to obtain a homogenate liquid, cooling after sterilization, B. preparing a mother starter, taking a dairy solution, cooling after sterilization, and putting it in the dairy solution Add compound lactic acid bacteria suspension, anaerobic culture to obtain the mother starter, C, prepare fermentation broth, put the fermentation base material under sterile conditions, adjust the temperature to 35-42°C, then add an appropriate amount of mother starter, and cultivate for 3-6 Hours, D, hydrolyzed, the temperature of the fermented liquid is raised to 45-55°C, hydrolyzed at 45-55°C to obtain a hydrolyzed liquid, E, dried, an appropriate amount of cyclodextrin is added to the hydrolyzed liquid, dried to remove moisture, and dried powder. The invention has rich nutrition, good taste and is easy to be absorbed by human body.