Abstract: Objective: To analyze the changes in phenolic composition and antioxidant function of mulberry beverage before and after fermentation. Methods: Mulberry beverage was prepared by a secondary fermentation method using Pichia kudriavzevii and Weissella confusa. The scavenge rate and reducing power of phenolic substances on DPPH, OH, ABTS⁺ free radicals before and after mulberry fermentation were compared. The differential metabolites in different samples were identified using non targeted metabolomics techniques. Then the correlation between changes in phenolic composition before and after fermentation and antioxidant performance was further explored. Results: The free radical scavenging rates of mulberry phenolic substances (1 mg/mL) on DPPH increased from 45.97%±2.23% before fermentation to 59.01%±3.59% after fermentation (P<0.05), while the scavenging rates on OH increased from 68.68%±1.23% to 78.55%±0.57% after fermentation (P<0.05). The reducing power of phenolic substances in fermented mulberry beverages was significantly increased (P<0.05). There was no significant difference in the ability of phenolic compounds to scavenge ABTS⁺ radicals before and after fermentation (P>0.05). A total of 46 differential metabolites were screened from mulberry phenolic substances before and after fermentation, including 3 anthocyanins, 10 phenolic acids, 18 flavonoids, and 15 other phenolic substances, of which 26 metabolites were up-regulated and another 20 metabolites were down-regulated, involving a total of 4 related metabolic pathways, including the biosynthesis of secondary metabolites in plants, phenylpropanoid biosynthesis, flavonoid biosynthesis, and anthocyanin biosynthesis. 58.70% of phenolic substances were positively correlated with DPPH and OH radical clearance rates. Conclusion: Both the changes in phenolic composition and increased content of mulberry after fermentation result in a significant upregulation of antioxidant capacity.