Surface plasmonic effects have been widely used in photocatalytic reactions like CO₂ conversion in the past decades. However, owing to the significant controversy in the physical processes of plasmon photocatalytic reactions and difficulty in realizing CO₂ reduction, the influence mechanism of the plasmon effect on the CO₂ photoreduction is still under debate. In this study, Au particles deposited on various substrates were employed to acquire insights into the plasmon photocatalytic CO₂ reduction, including SiO₂, n-Si, p-Si, TiO₂–SiO₂, TiO₂-n-Si, and TiO₂-p-Si. It was found that the plasmon resonant enhancement (PRE) effect of Au–SiO₂ caused by the Au plasmon was stronger than that of Au–TiO₂–SiO₂ and Au-n-Si (Au-p-Si) in the visible-light range, while it was weaker for Au-n-Si (Au-p-Si) samples than Au–TiO₂-n-Si (Au–TiO₂-p-Si). The simulation results agree with the experimental conclusions. The photocatalytic results indicated that the catalytic activity of Au-n-Si (Au-p-Si) samples was lower than that of Au–TiO₂-n-Si (Au–TiO₂-p-Si), and Au–SiO₂ was lower than Au–TiO₂-SiO₂ and Au-n-Si (Au-p-Si) samples, suggesting that the direct electron transfer (DET) mechanism was dominant here compared with the PRE mechanism.