Photocatalytic H₂O₂ production using biomass-derived carbon catalysts is a sustainable strategy, yet the role of surface functional groups in governing reaction kinetics remains ineffectively explored. Herein, a series of hydrothermal carbon (HTCs) photocatalysts are synthesised from sucrose in five alcoholic solvents, with the ethanol-derived catalyst (Suc-EtOH) exhibiting a remarkable 85.9 % higher H₂O₂ yield than the water-derived counterpart (Suc-H₂O). Systematic characterization reveals that the Suc-EtOH possesses enriched surface hydroxyl groups, which are demonstrated to correlate positively with H₂O₂ production efficiency. Combined experimental and theoretical analyses elucidate the dual role of hydroxyls: (1) enhancing O₂ adsorption and (2) facilitating the oxygen reduction reaction (ORR). The universality of this alcohol-mediated hydroxylation strategy is further validated using diverse carbon precursors (starch, fructose, and cellulose), demonstrating its efficacy for simple biomass substrates. This work identifies a quantitative correlation between the surface hydroxyl density of Suc-EtOH catalysts and the kinetics of H₂O₂ generation. Furthermore, it provides theoretical guidance for the design of biomass-derived metal-free photocatalysts.