Abstract: To investigate the effect of lactic acid bacteria fermentation on acid reduction of cranberry juice and the changes of the main physicochemical indexes, active ingredients and antioxidant activity during the proces of acid reduction. In this study, Lactobacillus rhamnosus was used to ferment cranberry juice at 40 ℃, and pH, total acid degradation rate, color difference, total soluble solids (TSS), total sugars, total polyphenols, total flavonoids, anthocyanins, γ-aminobutyric acid (GABA), vitamin C, and total amino acids of cranberry juice were determined. DPPH, ABTS⁺ free radical scavenging ability and FRAP iron ion reducing power were used to evaluate the antioxidant activities in vitro, and Pearson correlation analysis was conducted between the main components and their antioxidant activities. The results showed that the total acid degradation rate of cranberry juice reached the highest value (28.34%) on the 5th day of fermentation, indicated that lactic acid bacteria fermentation could significantly reduce the acidity of cranberry juice (P<0.05), and the contents of TSS, total sugars and anthocyanins showed a decreasing trend, the color difference showed an upward trend, while the contents of total polyphenols and total flavonoids increased first and then decreased, their highest values were 3.89 and 2.56 mg/mL, respectively. The contents of GABA, vitamin C and total amino acid basically showed a continuous increasing trend, and the antioxidant activity indexes of fermented cranberry juice increased first and then decreased, and the antioxidant activity was significantly higher than that of unfermented group (P<0.05). The maximum values of DPPH and ABTS⁺ free radical scavenging capacity and FRAP iron ion reduction capacity reached 84.30%, 74.68% and 703.08 mmol/L, respectively. Correlation analysis showed that total polyphenols, total flavonoids, GABA, vitamin C and total amino acids were significantly positively correlated with their antioxidant activities (P<0.05). This study provided a new idea for the acid reduction process of cranberry juice, and also provided theoretical basis and technical parameters for its large-scale production.