To investigate the mechanisms underlying the effects of Lactobacillus plantarum HC1775 fermentation on sugar and acid composition and antioxidant activity in litchi juice, this study analyzed the changes in sugar and acid composition, total phenolic content, and antioxidant activity during the fermentation process. Enzyme activity analysis of the juice and structural characterization of the pomace were also performed to elucidate the potential mechanisms of polyphenol release. During the 0~48 h fermentation period, the total sugar content of litchi juice decreased from 164.73 to 87.81 mg•mL-1, which was mainly caused by changes in sucrose content, with its level decreasing from 95.38 to 44.98 mg•g-1, while reducing sugar content decreased from 55.99 to 40.51 mg•mL-1. The content of lactic acid, which was the main acid produced by the fermentation, increased significantly from 0.17 to 12.10 mg•mL-1. Tartaric acid content increased from 1.62 to 2.05 mg•mL-1, whereas citric acid content decreased from 2.18 to 0.13 mg•mL-1, and ascorbic acid content decreased from 21.20 to 11.71 mg/100 g. In addition, the total phenolic content of litchi juice increased significantly, reaching a maximum of 293.4 mg•L-1. The changes in antioxidant activity followed a similar trend, with the DPPH radical scavenging rate increasing by 19.64% and the FRAP value increasing by 51.11%. This was mainly owing to elevated activities of cell wallrelated enzymes, which disrupted hydrogen and glycosidic bonds in cellulose and promoted the release of bound phenolics, thereby enhancing the antioxidant activity of litchi juice. Therefore, this study explains changes in sugar and acid composition and the potential mechanisms underlying changes in antioxidant activity in fermented litchi juice, thereby providing a theoretical basis for fermented litchi juice products.