Based on ancient records of “Marching tea” and the flavonoid-rich properties of Lespedeza juncea, this study optimized green tea processing parameters for Lespedeza juncea tea. By adjusting leaf loading, roasting temperature, and time and employing precise methods such as gas chromatography–olfactory–mass spectrometry (GC-O-MS), liquid chromatography–mass spectrometry (LC-MS), and electronic tongues to determine key indicators, including metabolites, volatile compounds, and flavor profiles, the optimal processing conditions were determined through comprehensive analysis as 150 g, 200 °C, and 180 s. These conditions resulted in superior sensory quality (score: 90.9), the highest flavonoid content (4.11%), strong antioxidant activity (DPPH: 66.0% at 20 mg/mL; ABTS: >95% at 2.5 mg/mL), and key aroma compounds 1-octen-3-ol and β-ionone. This work revives an ancient tea tradition and provides a scientific basis for developing novel functional teas from grassland resources.