As a fruit with both medicinal and edible value, red jujube (Ziziphus jujuba Mill.) is abundant in polyphenols and other bioactive components, exhibiting potent antioxidant potential. However, fresh jujubes are highly susceptible to spoilage, while existing processed products suffer from low added value and severe homogenization, restricting the upgrading of the jujube deep-processing industry. In this study, a strain suitable for jujube juice fermentation was selected from six lactic acid bacteria (LAB) strains. Using single-factor tests and response surface methodology, the fermentation conditions of fermented red jujube juice (FRJJ) were optimized. Systematic analysis was performed on the dynamic evolution of physicochemical properties, color, flavor, and bioactive compounds throughout the fermentation process. Additionally, a C2C12 cell model subjected to H₂O₂-induced oxidative stress was employed to evaluate the cytoprotective effects and antioxidant activity of FRJJ. The results demonstrated that LAB fermentation increased superoxide dismutase activity and total phenolic content, while improving flavor and sensory quality. Furthermore, FRJJ effectively mitigated oxidative stress damage and enhanced cell viability by reducing intracellular reactive oxygen species levels and maintaining mitochondrial membrane potential. These findings offer a theoretical basis for the value-added utilization of jujube resources and the formulation of functional fermented beverages.