Density functional theory is employed to investigate the plasmon-driven CO2 reduction at the active sites of metallic silver clusters. The results predict that CO2 prefers to adsorb at the bridge site of silver clusters and the C-O bond is difficult to break due to a high activation barrier at the electronic ground state. However, as the photogenerated plasmon energy of silver nanoparticles matches with the dissociation energy of the C-O bond, the CO2 easily dissociates into CO and an adsorption oxygen atom. Moreover, our calculated results demonstrate that the generated CO strongly adsorbed on silver clusters is favorable to its succeeding reduction reaction with hydrogen to CH3OH and CH4. The reaction barrier of the generation of CH3OH is lower than that of the formation of CH4, because the proton combines with the carbon more easily than with the oxygen atom at the initial reaction step. It well accounts for the experimental observation that CH3OH can be formed from the CO2 reduction on silver nanoparticles under visible light irradiations.