The aroma of fermented milk products is a key determinant of consumer preference. This study investigates the impact of different lactic acid strains on the aroma characteristics of fermented milk, identifies key volatile compounds, and establishes odor molecule labels to guide strain selection and modification. Sensory evaluation, dynamic headspace sampling (DHS), and gas chromatography olfactometry–mass spectrometry (GC-O-MS) were used to analyze 23 milk samples prepared with various lactic acid bacteria strains. A total of 74 volatile compounds were identified by GC-O-MS. Fermented milk P4 had the highest total volatile compound content (1566.50 ng/g). Flavor profiles were found to depend on strain metabolism rather than specific genera, with fermentation flavor quality enhanced by strains containing 2,3-butanedione, acetic acid, and sulfur compounds. Four distinct flavor clusters were established through molecular labels. These results provide targeted guidance for industrial strain selection and modification in fermented milk production, enhancing sensory appeal and consumer acceptance.