Off-gas analysis plays an important role in the production of fermented foods. It is noninvasive and enables on-line process monitoring of the physiological status of the fermentation, including growth kinetics and substrate consumption. It also helps determine the optimum point to halt the process for maximum yield. Two kinds of analytical method，as well as the applications in fermented food such as amino acid, organic acid, enzymes and so on were introduced. Process mass spectrometer will be potentially used in various of fermented food production fields, which can monitor several fermenters simultaneously with long-term good stability and high-precision.