Conversion of CO2 into value-added fuels and chemicals faces the challenges of high overpotential, low selectivity for desired products, and sluggish multi-electron transfer kinetics. Plasmon-mediated electrocatalytic methods show potential for overcoming these problems. In this work, choosing Au nanoparticles (NPs) as the dual-functional material of the electrocatalyst and plasmonic support, methanol can be efficiently produced from CO2 reduction. The dependences of current density and Faradaic efficiency (FE) on the Au NP size, light wavelength, and light density are systematically investigated. The performance test results show that CH3OH with FE as high as 52% can be obtained at a potential of -0.8 V (vs RHE) when a 20.2 nm Au NP-modified glass carbon electrode is under excitation using 520 nm light with a light density of 120 mW/cm(2). The performance results, along with the discrete dipole approximation calculation and electrochemical characterizations, suggest that the efficient activation of CO2 by plasmon-generated energetic electrons accounts for the enhanced conversion rate and the high selectivity for CH3OH production.