Actual chemical states of single-atom metal on reducible supports remain a fiercely debated topic under reactive environments. Herein, we demonstrate that the single-atom Pt on Co₃O₄ surface undergoes an in-situ reconstruction to form isolated Pt-Co bimetallic sites via reducing coordination number of Pt–O in the presence of hydrogen from both simulations and in-situ X-ray photoelectron spectroscopy. The modified chemical states of Pt greatly promoted H₂ activation, thus delivering a significantly high turnover frequency of 7,448 h⁻¹ (19.5 times over Pt nanoparticles on Co₃O₄) for hydrogenation of cinnamaldehyde. The satisfactory selectivity of 95.2% towards cinnamyl alcohol was ascribed to a tilted adsorption configuration of reactant on the catalyst surface via aldehyde group. We anticipate that the recognitions on in-situ reconstruction of single-atom catalysts (SACs) under the reducing conditions benefit the design of highly-performed hydrogenation catalysts.