Abstract A fundamental challenge in CO2 photoreduction is to establish highly efficient photocatalysts with efficient charge separation, wide-spectrum absorption and effective CO2 adsorption. The former two can be achieved by fabricating Z-scheme systems with narrow-bandgap semiconductor, and the last can be realized by creating vacancy defects in the catalyst. Herein, ZnSe/CdSe composites with different ZnSe/CdSe ratios are prepared via epitaxial growth of CdSe on ZnSe nanoparticles, which exhibit much higher CO2 photoreduction performance than pristine ZnSe under visible-light irradiation. ZnSe/CdSe (precursor ratio Zn:Cd = 1:0.125) exhibits an optimal CO yield (116.9 μmol g−1), which is 33.4 times that of pristine ZnSe (3.5 μmol g−1). Electron spin resonance (ESR) and density functional theory (DFT) calculations reveal that charge transfer at the ZnSe/CdSe interface follows Z-scheme pathway. Improved light harvesting by loading CdSe can further promote charge generation. Se vacancy generated during the preparation can facilitate CO2 adsorption.