Abstract: Yellow serofluid of tofu was used as a raw material, organic acid coagulant was prepared by Lactobacillus bulgaricus and Streptococcus thermophilus fermentation. The influences of lactic acid content were investigated by initial p H, lactose content, fermentation time culture temperature and inoculate quantity.Orthogonal experiment was used to optimize carbon source and its fermentation condition. The optimize culture conditions of organic acid coagulant were showed as follows:lactose content 1.5%, initial p H =6.5, inoculate quantity 6%, culture temperature 37℃, fermentation time 84 h. The lactic acid content under these conditions was 5.1g/L. Under the above process conditions, the rate of production, hardness, cohesiveness and protein content were 196.43%, 419.15 g, 261.42 gs and 9.62% respectively. Lactic acid content was increased and function of coagulant was improved by compare with other methods, protein content of tofu was also increased.