The　conversion　of　CO2　into　renewable　fuels　by　artificial　photosynthesis　is　an　attractive　solution　for　storing　solar　energy　in　the　form　of　chemical　fuel.　Herein,　we　presented　a　general　molten-salt　route　to　synthesize　spinel　germanium　and　iron　double-substituted　ZnGa2O4　solid　solutions.　Introducing　ZnFe2O4　and　Zn2GeO4　into　ZnGa2O4　can　effectively　expand　the　light-harvesting　wavelength　range　to　improve　the　ability　of　photocatalyst　in　CO2　reduction　and　H2O　oxidation.　The　solid　solutions　provide　a　larger　effective　mass　of　holes　compared　with　electrons　through　the　introduction　of　Fe3d,　Ge4s　and　Ge4p　orbitals　according　to　DFT　analysis.　This　leads　to　a　great　difference　in　the　mobility　between　the　electrons　and　holes　to　lessen　the　electron-hole　recombination　rate,　and　enhance　the　conversion　of　CO2　and　H2O　in　kinetics.　This　approach　is　developed　to　achieving　smaller　bandgap　values　and　a　closer　bracketing　of　the　CO2/reduced-H2O/oxidized　redox　couples　to　drive　the　overall　conversion　of　CO2　and　H2O.