This study combined quantitative descriptive analysis (QDA) and liquid chromatography-triple quadrupole tandem mass spectrometry (LC-TQ-MS/MS) to elucidate the effects of initial processing methods and roast degree on the content of major phenolic acid components and astringency intensity in coffee. Based on the interaction between phenolic acids and salivary proteins (α-amylase (AMY)), it revealed the molecular mechanism by which phenolic acids induce oral astringency. Results showed that light-roasted coffee contained higher levels of phenolic acids and greater astringency. Dose-over threshold (Dot) and correlation analyses identified chlorogenic acid, cryptochlorogenic acid, neochlorogenic acid, and isochlorogenic acid B (Iso B) as the major contributors to inter-sample variations in astringency intensity. Turbidity, particle size, and zeta potential analyses demonstrated that the Iso B-AMY complex exhibited the largest particle size and the lowest stability, suggesting its susceptibility to protein aggregation and astringent perception. Isothermal titration calorimetry and molecular docking clarified that the binding of astringent phenolic acids to AMY was mainly driven by hydrophobic interactions and hydrogen bonds. Among them, Iso B had the lowest binding energy (–7.45 kcal/mol) and the lowest dissociation constant (11.80 × 10^–6 mol/L) with AMY, indicating that Iso B was the key phenolic acid. This study systematically identified the key astringent phenolic acids in coffee, and clarified the structure-activity and dose-effect relationships of coffee phenolic acids in relation to their astringency intensity, laying the foundation for understanding the relationship between the sensory characteristics and chemical components of coffee.