Abstract: In order to study the effect of compound lactic acid bacteria fermentation on reducing acid of sea buckthorn juice, the Oenococcus oeni and Lactobacillus brevis were used to ferment sea buckthorn juice. With the total acid degradation rate as the index, the fermentation process of sea buckthorn juice was optimized by single factor experiment and response surface experiment, and the changes of flavonoids, polyphenols, total acid, pH, total soluble solids (TSS), reducing sugar, organic acid and antioxidant activity during fermentation were studied. The results showed that the optimal fermentation conditions were as follows: The ratio of Oenococcus oeni to Lactobacillus brevis was 1:1, the initial pH was 3.7, fermentation temperature was 31 ℃, fermentation time was 18 h, inoculation amount was 5%. Under these conditions, the total acid degradation rate reached 38.52%. At this time, the contents of flavonoids, polyphenols, total acid, pH and TSS were significantly different from those before fermentation (P<0.05). The contents of flavonoids and polyphenols in sea buckthorn fermentation broth increased by 44.74% and 22.22%, respectively, compared with those before fermentation. The pH increased from 3.71 to 3.85, the total acid content decreased from 8.49 g/L to 5.22 g/L, the TSS content decreased by 20.29%, and the reducing sugar content did not change significantly (P>0.05). The content of malic acid in fermentation broth decreased significantly (P<0.05), the degradation rate was 94.59%, and the content of lactic acid increased significantly (P<0.05), reached 10.37 mg/mL. The changes of quinic acid, ascorbic acid, tartaric acid, oxalic acid and citric acid were not significant (P>0.05). The maximum values of DPPH and ABTS⁺ free radical scavenging rate and ferric reducing antioxidant power (FRAP) of sea buckthorn juice after lactic acid fermentation reached 78.20%, 64.48% and 1.1626 mmol/L, respectively. Therefore, the sea buckthorn juice fermented by compound lactic acid bacteria could effectively reduce its acidity and improve its quality and antioxidant activity, which would provide a theoretical basis for the development of acid-reducing technology and related products.