Cu2O nanocubes decorated with Ag nanoparticles were synthesized based on a one-step wet chemistry approach. The content of Ag nanoparticles was adjustable by tuning the AgNO3 concentration. The photocatalytic performances of the Ag/Cu2O nanocubes were systematically evaluated, and the influences of the Ag nanoparticles were analyzed. The introduction of Ag nanoparticles reduced the band gap, increased the photocurrent and most importantly enhanced the photocatalytic activity towards organic pollution degradations under visible light illumination. There is an optimum content of Ag nanoparticles for the Ag/Cu2O nanocubes to achieve the best performances. A model based on the band structure theory is proposed to explain the excellent performance of the Ag/Cu2O nanocubes.