Abstract: The fermentation of lactic acid bacteria can enhance the transformation of active substances such as polyphenols in food, improving their functional activities. However, how lactic acid bacteria fermentation affects phenolic compounds in Dendrobium officinale (DO) is not clear. This study selected three common lactic acid bacteria to ferment DO from five regions. Then, the total phenolic and flavonoid contents and the antioxidant activity of DO before and after fermentation were determined. The study also analyzed the phenolic components and metabolite composition using high-performance liquid chromatography（HPLC） and ultra-performance liquid chromatography-tandem mass spectrometry（UPLC-MS/MS）. The results showed that as fermentation progressed, the viable cell counts in the DO fermentation broth gradually increased, while the pH value gradually decreased. The growth rate of viable cells was highest during the first 0 to 24 hours of fermentation, as was the rate of decline in pH. All three lactic acid bacteria increased the total phenolic and total flavonoid contents and antioxidant activity of DO. The most significant enhancement was observed in DO in Wenshan, Yunnan, fermented with Lactobacillus bulgaricus for 48 hours. Under this condition, the total phenolic and total flavonoid contents of the fermented DO were 120.67 mg/g and 23.39 mg/g, respectively. Additionally, the radical scavenging abilities against the cationic ABTS radical (2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) and DPPH (1,1-diphenyl-2-picrylhydrazyl) as well as the ferric reducing ability (FRAP) were highest at 288.51, 34.34, and 136.32 mg/g, respectively, which were increases of 0.81, 2.11, and 0.57 times compared to before fermentation. The DO fermented under this condition was selected for quantitative analysis of 16 phenolic compounds using HPLC. Significant changes were observed in four compounds before and after fermentation. Specifically, the content of sinapic acid increased from 0.65 mg/g to 1.14 mg/g (an increase of 75.38%), while vanillic acid rose from 2.60 mg/g to 3.19 mg/g (an increase of 22.70%). However, the content of (-)-epicatechin gallate decreased from 2.92 mg/g to 0.47 mg/g (a decrease of 83.90%). Moreover, salicylic acid was only detected in the post-fermentation samples. Non-targeted metabolomics analysis revealed that a total of 1,298 metabolites were identified from Dendrobium officinale before and after fermentation. Among these, 100 and 85 metabolites were significantly upregulated in positive and negative ion modes, respectively, while 211 and 180 metabolites were significantly downregulated. The number of upregulated metabolites was notably lower than that of downregulated metabolites. The upregulated metabolites were primarily concentrated in fatty acids, nucleosides, and alkaloids, whereas the downregulated metabolites were mainly found in flavonoids, phenolic compounds, and terpenes. This indicates that these metabolites may be gradually consumed or metabolically transformed during the fermentation process. Correlation analysis demonstrated a high correlation between antioxidant activity and phenolic compounds. Enrichment analysis of metabolic pathways during Dendrobium officinale fermentation using the Kyoto Encyclopedia of Genes and Genomes (KEGG) suggested that the ABC transport pathway, biosynthesis of various secondary metabolites - part II and metabolism of alanine, aspartate, and glutamate were the most likely metabolic pathways. In conclusion, the fermentation of lactic acid bacteria, especially Lactobacillus bulgaricus, significantly improved the antioxidant activity of DO, which may be related to the transformation and release of phenolic compounds during fermentation. The results of this study can provide a scientific basis for the development and utilization of Dendrobium officinale, offering insights into the biotransformation of active components in natural products/foods.