Abstract: Using mulberry as raw material, single and mixed strains of Lactobacillus plantarum and Bifidobacterium longum were used to ferment mulberry juice, and single-factor and response surface tests were used to investigate the optimal fermentation process of fermented mulberry juice, and to determine and analyze the functional components (total flavonoids, total anthocyanins, total phenols) and antioxidant activities (ABTS⁺ free radical scavenging rate, DPPH free radical scavenging rate, hydroxyl radical scavenging rate, total antioxidant capacity), etc. The results showed that the optimal fermentation process for fermented mulberry juice was 0.06% strain addition, initial pH6.1, fermentation temperature 37 ℃, and 0.09% oligofructose addition. The fermentation of lactic acid bacteria improved the functional components and antioxidant activity of fermented mulberry juice. Compared with single strain and mixed strain, the fermentation of mixed strain significantly increased (P<0.05) the content of functional compounds in mulberry juice, and the contents of unfermented mulberry juice and mixed strain after 48 h fermentation were 4.18~6.36 mg/mL of total flavonoids, 0.67~1.95 mg/mL of total anthocyanins, 12.62~18.65 mg/mL of total phenols. And 48 h fermentation of the mixed strains significantly improved the antioxidant activity of mulberry juice, with ABTS⁺ free radical scavenging rate 61.81%~88.17%, DPPH free radical scavenging rate 52.78%~81.64%, hydroxyl radical scavenging rate 37.38%~86.07%, total antioxidant capacity 17.85~29.49 mmol/L. The fermentation of Lactobacillus plantarum and Bifidobacterium longum mixed strains had better organoleptic properties. The study can provide a theoretical reference for the deep processing of mulberry.