Fermentation， contributing to convert polyphenol glycosides to their respective aglycones， could improve their bioavailability as well as flavor of the product. This study performed Lactobacillus plantarum YHG1-155 fermentation of sea buckthorn juice (SBJ) to analysis the changes of its antioxidant activity in vitro and explore the biotransformation process of polyphenols in the fermentation. The results showed that L. plantarum fermentation increased the viable cell count， free phenolic content， total phenolic content， and antioxidant activity of SBJ. In total， 116 significantly different metabolites were identified in L.plantarum fermented SBJ using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Among them， 67 (53 upregulated and 14 downregulated)， 64 (32 upregulated and 32 downregulated)， and 116 (74 upregulated and 42 downregulated) kinds of differential metabolites were identified between the groups Y2 and Y4， Y4 and Y8， and Y2 and Y8. Furthermore， KEGG pathway enrichment analysis showed that 18 kinds of differential metabolites between Y4 and Y2 and 13 kinds of differential metabolites between Y8 and Y4 were mainly involved in three pathways， namely the biosynthesis of secondary metabolites， flavonoid， flavone and flavonol biosynthesis pathways. Among them， the biosynthesis pathway of secondary metabolites was an important pathway for polyphenol biotransformation， and polyphenol glycosides might mainly be converted to their respective aglycones through this pathway. Our findings revealed the biotransformation of polyphenol glycosides of SBJ by L. plantarum YHG1-155 fermentation， providing a theoretical reference for the deep processing of hippophae rhamnoides.