As a promising plasmonic photocatalyst under visible-light irradiation, Au/TiO2 nanocatalyst commonly suffers from risk of Au nanoparticles aggregation and relatively long activation period by using the conventional calcination activation method. We report here that a short-term O-2 plasma treatment enables the Au/TiO2 catalyst to exhibit higher visible-light photocatalytic activity for formaldehyde (HCHO) oxidation compared with the calcination activated sample. X-ray photoelectron spectroscopy characterization suggests that Au/TiO2 catalyst activated by O-2 plasma achieves metallic Au content of 72.4% and surface oxygen species content of 29.8%, which not only lead to rapid reduction of cationic Au in HCHO oxidation but also facilitate the preliminary oxidation of HCHO to dioxymethylene and formate on catalyst surface. UV-vis diffuse reflectance spectra, photoluminescence spectra and diffuse reflectance infrared Fourier transform spectra measurements indicate that O-2 plasma activated Au/TiO2 catalyst presents an enhanced visible-light absorption and surface charge transfer efficiency in HCHO oxidation, which accelerates the formation of superoxide (O-2(-)) and OH center dot radicals via hot electron transfer mechanism under visible-light irradiation, remarkably promoting formate oxidation to carbonate, carbonate decomposition and thus HCHO oxidation.