Porous CuO micro-/nanostructures with clean surface, prepared through Cu2(OH)2CO3 precursor followed by calcination in air, were proven to be an effective peroxidase mimic. They can quickly catalyze oxidation of the peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H2O2, producing a blue color. The obtained porous CuO micro-/nanostructure have potential application in wastewater treatment. The apparent steady-state kinetic parameter was studied with TMB as the substrate. In addition, the potential application of the porous CuO in wastewater treatment was demonstrated with phenol-containing water as an example. Such investigation not only confirms the intrinsic peroxidase-like activity of micro-/nanostructured CuO, but also suggests its potential application in wastewater treatment.