Photocatalytic CO₂ reduction driven by green solar energy could be a promising approach for the carbon neutral practice. In this work, a novel defect engineering approach was developed to form the Sn_xNb_1−xO₂ solid solution by the heavy substitutional Nb-doping of SnO₂ through a robust hydrothermal process. The detailed analysis demonstrated that the heavy substitution of Sn⁴⁺ by a higher valence Nb⁵⁺ created a more suitable band structure, a better photogenerated charge carrier separation and transfer, and stronger CO₂ adsorption due to the presence of abundant acid centers and excess electrons on its surface. Thus, the Sn_xNb_1−xO₂ solid solution sample demonstrated a much better photocatalytic CO₂ reduction performance compared to the pristine SnO₂ sample without the need for sacrificial agent. Its photocatalytic CO₂ reduction efficiency reached ~292.47 µmol/(g·h), which was 19 times that of the pristine SnO₂ sample. Furthermore, its main photocatalytic CO₂ reduction product was a more preferred multi-carbon (C₂₊) compound of C₂H₅OH, while that of the pristine SnO₂ sample was a one-carbon (C₁) compound of CH₃OH. This work demonstrated that, the heavy doping of high valence cations in metal oxides to form solid solution may enhance the photocatalytic CO₂ reduction and modulate its reduction process, to produce more C₂₊ products. This material design strategy could be readily applied to various material systems for the exploration of high-performance photocatalysts for the solar-driven CO₂ reduction.