Copper oxide (CuO) was prepared through a facile one-step hydrothermal method and used as an activator of peroxymonosulfate (PMS) for the degradation of azo dye Acid Orange 7 (AO7), as a model organic pollutant. The effects of several parameters on the decolorization efficiency on AO7 were investigated. The degradation kinetic characteristics were studied by investigating the effect of different operational parameters including CuO dosage, pH and initial AO7 concentration. The reactive radical species were determined indirectly by way of radical quenching tests, as well as directly by in situ electron paramagnetic resonance technique spin trapping tests. In a 15 min reaction using 5 mM PMS, a CuO dosage of 0.1 g/L, at pH 6.65, 95.38% of 40 mg/L AO7 can be removed. This study provides an effective oxidative system for AO7 removal by CuO/PMS heterogeneous process at ambient temperature.