Here we examine Ag-Cu2O core-shell nanoparticles with bumpy structures for use in plasmonic photocatalysts. We synthesized the nanoparticles using a very simple co-reduction process. The shell thickness of the nanoparticles was controlled in order to investigate the effects of shell thickness on photocatalytic activity and stability. With increasing shell thickness, it was be observed that the activity and the stability were simultaneously improved. A thin shell lead to de-wetting of Cu2O from the Ag core and considerable oxidation of Cu2O to CuO. The high activity could be due to the unique structure of Ag-Cu2O, which has a high surface area and plasmonic charge transfer from the Ag core. In addition, we elucidated the stability tendency using conducted density functional theory (DFT) calculation. Strain induced between the Ag core and shell is critical to the stability, leading to de-wetting and oxidation of Cu2O.