Abstract: This study aimed to systematically evaluate the impact of lactic acid bacteria fermentation on the flavor quality of birch sap, providing a scientific basis for the selection of strains for fermented birch sap. Eight distinct strains of lactic acid bacteria were selected to ferment birch sap collected at various intervals. The study comprehensively analyzed alterations in physicochemical properties, antioxidant activity, amino acid composition, and volatile compounds of birch sap before and after fermentation. Results indicated chemical composition disparities in birch sap collected at different times, with early sap exhibiting higher sugar and acid levels, a more diverse array of volatile compounds, yet lower content of flavor amino acids compared to the late sap. The eight strains had good growth and metabolism in birch sap. After fermentation by the strains, the pH and reducing sugar content of birch sap decreased, while the total acid content increased. The antioxidant capacity of birch sap was enhanced. Through analysis using high-performance liquid chromatography, a total of 21 amino acids were detected in fermented birch sap. After fermentation, both sweet and bitter amino acid levels rose in early sap, contrasting with a decline in sweet amino acids and relatively stable bitter amino acid levels in late sap. The composition of volatile compounds was analyzed using headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry. A total of 46 volatile compounds were detected in birch sap, with both the variety and quantity of volatile compounds experiencing significant enhancement post-fermentation. Notable variations in quality transformations were observed in birch sap collected at different time points following fermentation. The changes in total acid content, reducing sugar content, amino acid composition, and volatile compounds of fermented birch sap collected in early sap were more significant. Principal component analysis revealed distinct quality differences between fermented and unfermented birch sap. A comprehensive review shows that lactic acid fermentation can enhance the flavor quality of birch sap, and Lactiplantibacillus plantarum BB16 is considered the optimal fermentation strain.