The invention discloses a preparation method of fermented soya-bean milk, and belongs to the technical field of 
deep processing of agricultural products. The main 
raw material of the fermented soya-bean milk is mung beans. The method comprises the following steps: firstly, soaking washed mung beans in water for 10-15 hours according to the weight ratio of 1 to 8, performing pulping, standing 
slurry for 30-45 minutes, and performing filtering to finish 
powder-
slurry separation; pouring filtered 
slurry into a sterilized 
fermentation tank, sealing the tank with a sealing film, performing naturalfermentation in a 37 DEG 
C constant-temperature 
incubator for 6 hours, adding a proper amount of 
Lactobacillus sakei, and continuing to perform 
fermentation for 6 hours; after the 
fermentation is finished, adding proper NaHCO3 to adjust an 
isoelectric point so as to obtain raw bean juice; and finally, boiling the raw bean juice, and performing decocting with soft fire for 15-30 minutes to obtain the product. The preparation process disclosed by the invention solves the technical problems that traditional old Beijing fermented soya-bean milk processes have great dependence on natural conditions, the taste and 
flavor are difficult to unify, the fermentation time is long and the like. The method is simple and easy to operate and popularize, and the fermentation time is greatly shortened on the basis of reserving the traditional process; and the interference caused by the environment in the natural fermentation process of the bean juice is resisted to a certain extent. Therefore, the tasteof the bean juice is uniform, the texture is uniform, the 
palatability is good, the 
nutrition is rich, and the standardized production is favorably realized.