Abstract: This study was conducted to address the limitations of traditional fermented dairy products in flavor, nutrition, and functionality by introducing Limosilactobacillus fermentum FOSU YHD19 in synergy with a traditional starter culture. The effects of different inoculation concentrations (10⁵~10⁹ CFU/mL) on the quality of yogurt were systematically investigated. The fermentation process (including dynamic changes in pH, titratable acidity, water-holding capacity, and viable cell count) and storage stability were monitored, along with key indicators such as sensory properties, microstructure, and volatile flavor compounds after product maturation. The results demonstrated that the addition of FOSU YHD19 significantly enhanced fermentation performance, accelerating the acidification process (with the pH decreasing to 4.44 within 10 hours). It also effectively improved the texture characteristics of the yogurt by appropriately reducing hardness and elasticity, resulting in a denser and smoother mouthfeel. When the inoculation amount was 10⁷ CFU/mL, the fermented dairy product exhibited the best overall quality, outperforming the traditional starter culture. Specifically, the product showed high water-holding capacity, appropriate titratable acidity, a more compact and uniform protein gel network observed under microscopy, and a significantly higher sensory score compared to other groups. GC-MS analysis revealed an increase in the relative content of acids, alcohols, and ketones, imparting a richer fruity and cheesy aroma to the product. Furthermore, after 28 days of storage at 4 ℃, the viable bacterial count remained above 10⁶ CFU/mL, and all quality parameters remained stable. This study could provides both theoretical foundations and technical parameters for developing premium yogurt products with enhanced organoleptic properties and probiotic benefits.