Abstract: To address the problem of the sour and astringent taste of Rosa roxburghii Tratt. juice, in this study, we mixed fresh R. roxburghii juice with fresh apple juice, and then used Lactobacillus plantarum, Lactobacillus paracasei, and Lactobacillus fermentum, respectively, as starters for fermentation. Following fermentation, the physicochemical indices, in vitro antioxidant indices, organic acid contents, and volatile components of the fermented juices in each experimental group were determined, with the unfermented mixed juice being used as the control. The results of physicochemical analyses revealed that the total acid and tannic acid contents in the R. roxburghii–apple mixed juice fermented using L. paracasei were 3.61 g/L and 0.059 mg/mL, respectively, which were significantly lower than the respective values obtained for other experimental groups and the control group (P<0.05). For the L. paracasei fermentation, the contents of vitamin C and carotenoids were 2.75 mg/mL and 572.28 μg/mL, respectively, which were significantly higher than those recorded for the other experimental groups (P<0.05). In addition, the results of in vitro antioxidant assays revealed that fermentation with all three lactic acid bacterial strains promoted an enhancement of the antioxidant capacity of the mixed juice, among which, all assessed antioxidant indices of the L. paracasei-fermented juice were superior to the corresponding values obtained for the control group and other experimental groups. Moreover, the results of organic acid analyses indicated that among all groups, the L. paracasei group had the lowest malic acid content and highest lactic acid content, making it most conducive to improving the composition of organic acids in the mixed juice. The results of volatile component analyses indicated that fermentation with L. paracasei was most beneficial for retaining the key flavors of R. roxburghii and apple. In conclusion, among the assessed bacterial strains, using L. paracasei as a starter was found to be more beneficial for ameliorating the sour and astringent taste of the R. roxburghii–apple mixed juice, enhancing the antioxidant capacity of the juice, and retaining the key aroma substances.