Abstract: Using soy as the main material, the Lactobacillus bulgaricus ( L. bulgaricus) and Streptococcus thermophilus (S. thermophilus) with the activated and domesticated Lactobacillus rhamnosus (L. rhamnosus) was prepared by a centain percentage as mixed fermentation agent, through the single factor test and orthogonal experiment to determine the fermented soymilk production process. Results of testing to determine the three strains by single factor as work by 1∶0.5∶1 mixed. The order of affecting the fermentation of soymilk that the orthogonal test results showed were water ratio, inoculation, fermentation time and the amount of sugar added, the best conditions were:inoculum size of 5%, the material to water ratio in soybean yogurt of 1∶8, the addition of sugar 8%, 43℃ under the conditions of fermentation 5h. The conditions of production of soybean yogurt, with a rich flavor and delicate sweet and sour taste.