The solar‐driven catalytic reduction of CO2 to value‐added chemicals is under intensive investigation. The reaction pathway via *OCHO intermediate (involving CO2 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 C3N4 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 spectroscopy (CO stretches at 1637 cm−1). This study provides a unique example that controls the selectivity of the multi‐electron transfer mechanisms of CO2 photoconversion using heteronuclear dual‐atom‐site catalyst to generate an uncommon product (HCHO) of CO2 reduction. A heteronuclear dual‐atom site Sn(II) and Cu(I) photocatalyst embedded in the g‐C3N4 framework is fabricated to selectively produce formaldehyde from CO2 reduction under visible light. The optimized catalyst (Sn:Cu precursor mass ratio of 3:1) exhibits a high HCHO productivity of 259.1 µmol g−1 and a selectivity of 61% after 24 h visible light irradiation.