The protocooperation mechanisms of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilusis is of great importance to the fermentation of milk, especially to bacterial adaptability to in acid environment. This paper discusses the symbiosismechanisms of the two strains from the following perspectives: two-component signal transduction systems, horizontal gene transfer, protein metabolism, nucleotide base metabolism, oxidative stress and carbon source metabolism, aiming to provide valuable references for further investigations on improving the quality of fermented milk products.