Artificial photosynthesis, which converts carbon dioxide into hydrocarbon fuels, is a promising strategy to overcome both global warming and energy crisis. Herein, the geometric position of Cu and Ga on ultra-thin CuGaS 2 / Ga 2 S 3 is oriented via a sulfur defect engineering, and the unprecedented C 2 H 4 yield selectivity is ≈93.87% and yield is ≈335.67 µmol g −1 h −1 . A highly delocalized electron distribution intensity induced by S vacancy indicates that Cu and Ga adjacent to S vacancy form Cu-Ga metallic bond, which accelerates the photocatalytic reduction of CO 2 to C 2 H 4 . The stability of the crucial intermediates (*CHOHCO) is attributed to the upshift of the d-band center of ultra-thin CGS/GS. The C-C coupling barrier is intrinsically reduced by the dominant exposed Cu atoms on the 2D ultra-thin CuGaS 2 /Ga 2 S 3 in the process of photocatalytic CO 2 reduction, which captures *CO molecules effectively. This study proposes a new strategy to design photocatalyst through defect engineering to adjust the selectivity of photocatalytic CO 2 reduction.