<p>The structural complexity and inconsistent antioxidant performance of industrial lignin have long limited its high-value applications. In this study, we first clarified the central role of phenolic hydroxyl groups in antioxidant activity via targeted acetylation of tannin and lignin. Building on this insight, a green ethanol–water stepwise fractionation strategy (100%, 20%, 0%) was developed to deconstruct industrial lignin (KP) into three well-defined fractions (KE100, KE20, KE0) with tunable molecular properties. This solvent-gradient process enabled the orderly separation of high-molecular-weight hydrophobic components to low-molecular-weight hydrophilic ones. KE0, featuring the lowest molecular weight (M_w = 1000&#xa0;g/mol) and highest phenolic hydroxyl content (4.23 mmol/g), exhibited markedly enhanced antioxidant performance (DPPH: 62%; FRAP: 2.97 µmol/mg). These findings establish a clear structure–activity relationship and demonstrate that ethanol-based fractionation not only improves lignin homogeneity but also unlocks its potential as a sustainable antioxidant platform. This work offers a scalable, eco-friendly pathway for advancing lignin valorization toward functional material applications.</p>