Plasmonic metal nanocatalysts have excellent light trapping properties and high chemical reactivity. Impressively, Au nanostructures can absorb a wide array of visible light by tuning their morphology. In this work, spherical gold nanoparticles (Au NSs), multi-branched gold nanoparticles (Au NMs) and gold nanorods (Au NRs) were successfully synthesized; the shape- and size-dependences of these gold nanocatalysts on the methanol oxidation reaction (MOR) under light irradiation were studied. It is worth mentioning that Au NRs have the highest anode peak current density under dark conditions due to the exposure of highly active facets. A similar enhancement effect was obtained for Au NSs and Au NMs under visible light irradiation, which is due to the generation of a high concentration of energetic charge carriers on these Au nanostructures. The size dependences of Au NSs on the MOR showed that a larger electrochemically active surface area (ECSA) was obtained for small nanoparticles, which is due to the surface effect. In addition, the catalytic performance, durability and anti-CO stripping of these Au nanocatalysts under visible light irradiation, as well as the effect of light intensity and wavelength were described in detail. This work provides an insight into the mechanism of plasmon enhanced electrocatalysis by Au nanostructures with different sizes and shapes.