The solar-driven catalytic reduction of CO 2 to value-added chemicals is under intensive investigation. The reaction pathway via *OCHO intermediate (involving CO 2 adsorbed through O-binding) usually leads to the two-electron transfer product of HCOOH. Herein, a single-atom catalyst with dual-atom-sites featuring neighboring Sn(II) and Cu(I) centers embedded in C 3 N 4 framework is developed and characterized, which markedly promotes the production of HCHO via four-electron transfer through the *OCHO pathway. The optimized catalyst achieves a high HCHO productivity of 259.1 μmol g −1 and a selectivity of 61% after 24 h irradiation, which is ascribed to the synergic role of the neighboring Sn(II)–Cu(I) dual-atom sites that stabilize the target intermediates for HCHO production. Moreover, adsorbed *HCHO intermediate is detected by in-situ Fourier transform infrared (FTIR) spectroscopy (C=O stretches at 1637 cm −1 ). This work provides a unique example that controls the selectivity of the multi-electron transfer mechanisms of CO 2 photoconversion using heteronuclear dual-atom-site catalyst to generate an uncommon product (HCHO) of CO 2 reduction.