Streptococcus thermophilus is often used as a starter culture in the production of fermented dairy products. In this study， S. thermophilus IMAU80809 was selected as the research object and subjected to mutagenesis treatment with nitrosoguanidine (NTG). Through the determination of enzyme activities related to galactose metabolism and gene sequence alignment analysis， a mutant strain capable of efficiently metabolizing galactose， S. thermophilus IMAU80809Y， was screened out. On this basis， fermented milk of S. thermophilus IMAU80809 and its mutant strain was prepared， and the changes in pH value， titratable acidity， viscosity， water-holding capacity， viable cell count， and key metabolites were analyzed in detail. The results showed that compared with the wild type strain， the curdling time of the mutant strain's fermented milk was shortened to 7 h. The activities of β-galactosidase and galactokinase increased by 76.16% and 21.13% respectively. While the activities of pyruvate kinase and glucokinase decreased by 46.89% and 18.83% respectively. In addition， the metabolic amounts of lactose and galactose by the mutant strain increased by 0.599 g/100 g and 0.208 g/100 g respectively. These results indicate that the ability of the mutant strain to metabolize lactose and galactose was significantly higher than that of the wild-type strain. This study provides a theoretical basis for the production of low-galactose fermented dairy products.