The content of propionic acid in No. 1 stinky tofu detected by HPLC was about 1.32 mg/g， which was higher than the content of propionic acid in No. 2 stinky tofu from different manufacturers (0.56 mg/g)， and the content of propionic acid in tofu embryos before fermentation was very low， about 0.026 mg/g， but the content of propionic acid in brine was as high as 6.35 mg/mL. As a food additive， propionic acid must be marked on the packaging if it is added during the production process， but the No. 1 stinky tofu manufacturer did not artificially add propionic acid. Therefore， this experiment aimd to explore the causes and mechanisms of production of propionic acid during the fermentation of stinky tofu. Using metagenomics to analyze and identify the microbial composition in brine and stinky tofu， it was found that compared with the 'control brine' with very low propionic acid content， the species and abundance of Propionibacterium that produced propionic acid in No. 1 stinky tofu brine increased overall. Among them， the abundance of Propionibacteriaceae_bacterium_NML_160184 had increased by 7.6 times， the abundance of Acidipropionibacterium_acidipropionici acidigenes has increased by 4.7 times， and the abundance of Propionibacterium_bacteria has increased by 3.8 times. Compared with No. 2 stinky tofu， the species and abundance of Propionibacterium producing propionic acid in No. 1 finished tofu also increased significantly. The abundance of Propionibacteriaceae_bacterium P6A17 increased by 59.4 times， and the abundance of unclassified_f_Propionibacteriaceae increased by 54.9 times. The abundance of Propionibacteriales_bacterium increased by 47 times， and the abundance of Pseudopropionibacterium_massiliense increased by 29.7 times. Non-targeted metabolomics analysis of the microbial metabolites and functions in brine found that the metabolic activities of different brine microbial compositions were significantly different. The difference in propionic acid levels in stinky tofu was most likely due to the types and metabolic activities of the microorganisms in the brine. In summary， the detection of propionic acid in stinky tofu was due to the natural production of microorganisms during the fermentation process rather than artificial addition. Due to the different types and abundance of propionic acid-producing bacteria in brines from different sources， the propionic acid content of the final product will also showed significant differences.