The invention provides a mulberry fermentation product and a preparation method thereof. The preparation method comprises the following steps of performing mixed mashing on mulberries and water to obtain mashing liquid, adding a first carbon source to the mashing liquid to obtain mixed liquid, inoculating the mixed liquid with seed liquid, performing fermentation until the content of reducing sugar in the fermentation liquid is lower than 1%, and performing homogenizing and sterilizing on the fermentation liquid so as to obtain the mulberry fermentation product, wherein the seed liquid is obtained through the steps of inoculating culture mediums with lactobacillus brevis, lactobacillus paracasei, lactobacillus plantarum and streptococcus thermophilus, performing extended culture, and mixing the corresponding obtained enlarged-culture liquid, and the culture mediums comprise the following components of a nitrogen source, a second carbon source, inorganic salts, the mulberry pulp and thewater. Through the adoption of the preparation method provided by the invention, functional compositions in the mulberries can be sufficiently reserved.