To develop a novel black soybean yogurt drink, lactic acid bacteria (LAB) fermentation was used, and the fermentation process was optimized by single-factor experiments and response surface methodology, with viable LAB count as the key indicator. The effects of LAB fermentation on the nutritional composition, flavor, and antioxidant activity of black soybean yogurt were analyzed, and the fermented black soybean yogurt was subjected to simulated in vitro digestion to evaluate changes in antioxidant activity during the digestion process. The optimal conditions for fermenting black soybean yogurt were as follows: fermentation time of 34 h, fermentation temperature of 35 ℃, LAB inoculum size of 0.08%, and sucrose addition of 6.5 wt.%. After fermentation, the viable LAB count in black soybean yogurt was 9.27 log CFU/mL, total phenolic content increased to 311.75 mg.L-1, and total sugar content decreased to 90.25 mg.mL-1. The scavenging rate of fermented black soybean yogurt on 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals was up to 81.23%, and the scavenging rates for 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS+) cationic radicals and hydroxyl (.OH) radicals were 65.93% and 62.69%, respectively. After fermentation, the umami taste of black soybean yogurt increased from 0.42 to 6.82, and astringency decreased from -3.22 to -9.93. When the samples were subjected to simulated in vitro digestion until the endpoint, the DPPH and ABTS+ free radical scavenging rates in the fermentation group increased by 1.34-fold and 1.17-fold, respectively, compared with those in the non-fermented group. In conclusion, improvements in the nutritional composition, flavor, and antioxidant activity of black soybean yogurt can be achieved through LAB fermentation. These findings provide a theoretical basis for the development of other multifunctional LAB soybean milk products.