We demonstrated a facile and green preparation of cuprous oxide-reduced graphene oxide (Cu2O-rGO) nanoheterostructures through a photochemical reaction. The density of Cu2O nanocubes (NCs) grown on the rGO surface can be well controlled by modulating the concentration of GO employed in the reaction. Because of the relatively low potential of Fermi level of rGO, the photoexcited electrons on the conduction band (CB) of Cu2O NCs preferentially transfer to rGO, simultaneously leaving photogenerated holes on the valence band (VB) of Cu2O, resulting in the notable charge carrier separation properties. Time-resolved photoluminescence (TRPL) spectra were collected to quantitatively analyze the electron transfer rate constant (ket) between Cu2O NCs and rGO, and the dependence of the ket on the rGO constituent in Cu2O-rGO nanoheterostructures. Among all the samples tested, the Cu2O-rGO nanoheterostructure with the rGO constituent of 2 wt.% (denoted as Cu2O-rGO-2) displayed the largest ket as well as the most pronounced charge separation property. The optimized Cu2O-rGO-2 showed the best methyl orange (MO) photocatalytic degradation performance, which was highly consistent with the trend of the obtained ket results. As compared with relevant commercial products, such as N-doped P-25 TiO2 and commercial Cu2O powders, the Cu2O-rGO-2 exhibited superior efficiency toward MO degradation under visible light illumination, illustrating its potential for applications in relevant photoelectric conversion processes. The recycling trial showed that the Cu2O-rGO-2 has promising potential for use in the long-term course of photocatalysis to degrade organic pollutants. Furthermore, the photocatalytic efficiency evaluated under natural sunlight demonstrated that the present Cu2O-rGO nanoheterostructure could effectively harvest the energy of solar spectrum and converted it into the chemical energy for organic pollutants degradation. The current study could provide great insights into the design of semiconductor/graphene composites which exhibit remarkable charge separation properties for practical applications in the organic pollutants photodegradation, solar fuel generation as well as photovoltaic devices.