Photocatalytic degradation of acetaldehyde and its photocatalytic mechanisms over Au/TiO2 core-shell nano catalyst were, for the first time, investigated under UV and visible light irradiation. The results indicate that Au/TiO2 core-shell catalyst shows higher activity for the oxidation of acetaldehyde into CO2 under both UV and visible light irradiation comparing with P-25 and metal-deposited TiO2 photocatalysts. When Au/TiO2 core-shell catalyst is excited by UV light, the Au-core acts as the sink to restore the separated electrons, thus to improve the photoinduced charge separation; while under visible light irradiation, the mechanism can be understood as the coordinate effect of the plasmon resonance of Au-core particles and the formation of an impurity energy level induced by TiO2-xFx.