Photocatalytic conversion of CO2 into hydrocarbons by utilization of the solar energy is considered a promising approach to mitigate energy crisis and the environmental issues. Since the defects in a catalyst play an important role in CO2 reduction, herein, the porous ZnO nanoplates with vacancy defects are synthesized by annealing ZnS (en)(0.5) precursor in air at different temperature. The defect amount in ZnO changes with the annealing temperature, resulting in different photocatalytic activity for CO2 reduction. The related mechanism has been studied both experimentally and theoretically. Raman spectra and chemical composition of the obtained catalysts are used to determine the defects. Transient techniques are used to investigate the separation of photogenerated charge carriers. CO2 adsorption capacity for different catalysts is also measured. First-principles calculation is used to study the adsorption and activation of CO2 on the ZnO surface. We envision that this work may afford an efficient approach to develop the semiconductor photocatalysts with superior activity via defects engineering.