Elemental doping is an important method for improving the efficiency of catalysts. In this study, cobalt-doped CuO catalysts were prepared using a rapid precipitation method for the effective degradation of organic pollutants by photo-assisted activation of potassium peroxymonosulfate (PMS). The Co-doped CuO catalyst demonstrated degradation efficiency as high as 96% within 20 min reaction under visible light conditions, which is much higher than that of the undoped CuO. Cobalt doping transformed copper oxide nanoparticles from a three-dimensional needle-shape structure to a near-two-dimensional thin ribbon-like structure. Cobalt doping increases the flat-band potential of CuO and thus enhances the charge transfer efficiency. Furthermore, the effects of solution pH, initial dye concentration and catalyst dosage on the degradation efficiency were also investigated. XPS and EPR results show that cobalt doping can increase the oxygen vacancy content in CuO and thus improves the activation properties. The result of the trapping agent experiments indicates that main active species in the reaction are holes, while hydroxyl radicals, singlet oxygen, superoxide radicals and sulfate radicals are also involved. Finally, the reaction mechanism for the photo-assisted Co-doped CuO activated PMS degradation of RhB are generally proposed.