<p>Single-atom co-catalysts engineered on semiconductor substrates offer a cost-efficient pathway to improve the photocatalytic performance with minimal precious metal loading. However, the precise tuning of local coordination environments and the construction of efficient single-atom co-catalysts remain challenging for photocatalytic overall water splitting systems. In this work, we have employed an icing-assisted photochemical reduction strategy to anchor atomically dispersed Pt species as hydrogen evolution co-catalysts on Al³⁺-doped SrTiO₃ (Pt SA-STO) for photocatalytic overall water splitting. The optimized Pt SA-STO exhibits remarkable photocatalytic performance, achieving hydrogen and oxygen evolution rates of 13.62 and 6.71 mmol h⁻¹ g⁻¹, respectively, along with a turnover frequency (TOF) value of 2114.5 h⁻¹. We pioneer the application of nuclear magnetic resonance (NMR) spectroscopy to quantitatively characterize the temporal evolution of Pt⁴⁺ to Pt²⁺ under continuous irradiation during the icing-assisted photoreduction process; besides, advanced characterizations and theoretical calculations well evidence that single-atom Pt co-catalysts facilitate directional transfer and extraction of photogenerated charge carriers, effectively suppressing surface recombination of photogenerated electron-hole pairs. This work offers valuable insights into the design of novel single-atom co-catalysts by deepening the understanding of electronic configurations and active sites in photocatalytic overall water splitting.</p>