Tuning reactive metal-support interaction (RMSI) is a promising approach to optimizing catalytic active sites via the electronic, geometric and compositional effects. In general, the RMSI is conducted on the reducible oxides via a high-temperature reaction (>550 °C). Herein we report a strong RMSI between Pt single atom (PtSA) and non-oxide-based g-C3N4 built by an in-situ photocatalytic reduction method at a sub-zero temperature. The experimental observation confirms that the rich N vacancies in g-C3N4 produce an obvious electron-deficient effect, which greatly enhances the RMSI. This strong RMSI contributes to the highest PtSA coverage density of 0.35 mg m−2 reported to date in carbon-based materials and outstanding H2-evolution activity of 174.5 mmol g−1 h−1 per PtSA relative to those on the electron-rich g-C3N4. The structure simulation reveals that the RMSI can not only stabilize the PtSA on the electron-deficient g-C3N4 via the strong chemical bond between PtSA and the two-coordinated C (C2C) sites caused by the N vacancies, but also promises the PtSA with an optimized electronic and geometric structures for capturing photogenerated electrons and producing H2. This finding opens a new channel for designing and manipulating single atom-loaded photocatalyst via the RMSI at a sub-zero low temperature.