Theasinensin A (TSA), a dimeric flavan-3-ol from fermented teas, exhibits potent antioxidant activity exceeding that of its monomeric precursor, epigallocatechin gallate (EGCG). This study integrates experimental and computational methods (Density Functional Theory) to elucidate TSA's antioxidant mechanisms. DPPH assays revealed that TSA exhibited an IC50 value 2.4-fold lower than that of EGCG, indicating its superior radical scavenging efficacy. Electrostatic potential, frontier molecular orbital, and Laplacian bond order analyses identified the O4' - H group on the B-ring as the primary reaction site, with TSA showing higher reactivity than EGCG. Reaction enthalpy calculations further revealed that TSA requires less energy for double hydrogen donation, underscoring its thermodynamic advantage. Additionally, spin density analysis demonstrated the increased stability of TSA's phenoxy radical, which may be stabilized by its robust intramolecular hydrogen bonding, π-π stacking, and CH-π interactions. These findings highlight the unique structural and electronic properties of TSA that contribute to its superior antioxidant efficacy.