Abstract: Sauce-flavor Baijiu shows a typical sauce flavor, with a delicate mouthfeel. Its fermentation process consists of seven cycles, and each cycle produces one kind of base liquor. The fourth cycle liquor has a rich flavor and excellent quality. During the fermentation process, microorganisms are in a dynamic state, while flavor substances differ. This study investigated the microbial changes of Jiupei in the fourth cycle and their impact on flavor substances. Species diversity analysis was conducted on the fourth cycle Jiupei sample using high-throughput sequencing. Flavor substances were analyzed using headspace solid-phase microextraction combined with gas chromatography-mass spectrometry. Results showed that the dominant bacteria in the Jiupei were Lactobacillus, Virgibacillus, and Kroppenstedtia, while dominant fungi were Thermoascus, Aspergillus, and Issatchenkia. The microbial community in the Jiupei showed significant dynamic changes during the later stage of fermentation. The Jiupei showed the richest variety of alcohol and ester substances at the beginning of fermentation, with the relative content of alcohol, ester, and acid compounds showing a pattern of increasing firstly and then gradually decreased during fermentation. This study conducted a correlation analysis between fungi and bacteria at the genus level and flavor substances, revealing that Monascus, Lactobacillus, and Wickerhamomyces were positively correlated with key flavor substances, such as ethyl acetate, ethyl lactate, and ethyl hexanoate, respectively. The data provided a basis for comparing the microorganisms and flavor substances in the fourth cycle of sauce-flavor Baijiu, and offered a theoretical basis for improving the quality of Baijiu.