To compare the effects of different lactic acid bacteria fermentation on the quality and flavor of longan juice, Lactiplantibacillus plantarum, Lactobacillus bulgaricus, and Streptococcus thermophilus were used for single and mixed fermentation. Using the unfermented juice as control, the viable bacterial counts, pH value, and other physicochemical parameters of each group were determined, and the composition of volatile compounds was analyzed using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-ion mobility spectrometry (GC-IMS). The results showed that the single and mixed fermentation groups containing L. plantarum significantly decreased the pH value (from 6.98 to 3.60~3.63), and significantly increased the total acid content (from 1.96 g•L-1 to 6.78~6.87 g•L-1), while also showing the most pronounced degradation of soluble solids (from 10.00% to 9.13%~9.33%) and reducing sugars (from 36.69 to 24.88 mg•mL-1). The ternary mixed group (1:1:1) showed the highest viable bacterial count (8.80 log CFU/mL) and retention rate of total phenols (91.71%). Analysis of volatile components showed that fermentation transformed the monoterpene-dominated (90.74% of total) aroma lineage into a complex system composed of ketones, aldehydes, alcohols, esters, and other flavor compounds (GC-MS), with reduced off-flavor components, such as 2-hexenal and trans-2-heptenal (GC-IMS); the binary mixed group exhibited the greatest increase in both variety and total concentration of volatile components. In conclusion, ternary mixed fermentation (1:1:1) demonstrated synergistic effects in enhancing the viable bacterial counts, acidity, and total phenolic retention of longan juice, while binary mixed fermentation offered more advantages in the synthesis of flavor compounds. These findings provide a theoretical foundation for improving the quality and flavor of fermented longan juice.