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Solar-to-hydrogen energy conversion is a promising strategy to solve environmental pollution and energy crisis by utilizing photocatalytic water splitting. In this work, a "sandwich" structure of g-C3N4-based single-atom platinum (Pt) catalyst for photocatalytic water splitting is proposed and investigated using a combined first principles and semi-empirical study method. The calculation results indicate that, without any co-catalyst, the photogenerated holes in the valence band of BL-g-C3N4 cannot oxidize H2O to O2, and its oxygen evolution reaction (OER) performance is not better than that of the pristine monolayer g-C3N4. Significantly, the photogenerated holes in the valence band of the "sandwich"-structured photocatalyst g-C3N4–Pt1–g-C3N4 can oxidize H2O to O2 without any co-catalyst. That is, the OER performance of g-C3N4–Pt1–g-C3N4 is better than that of the pristine g-C3N4 and the pristine bilayer-g-C3N4 (BL-g-C3N4). However, it can be found that the introduction of single Pt atom confinement in the BL-g-C3N4 cannot effectively reduce hydrogen evolution reaction (HER) energy barrier or improve the hydrogen evolution kinetics of BL-g-C3N4. In other words, the introduction of the confined single Pt atom in the BL-g-C3N4 not only fails to improve HER performance of BL-g-C3N4, but deteriorates HER catalytic performance of BL-g-C3N4. These findings provide some theoretical insights for engineers to prepare photocatalysts with higher activity and stability. [ABSTRACT FROM AUTHOR]