Maintenance of monochloramine concentration during water disinfection is important to ensure the microbial safety of drinking water. The decay of monochloramine always occurs and some substances present in the water can accelerate this process. Copper often exists in ionic form in water, but the effect of Cu(II) on the decomposition of monochloramine is largely unknown. In this paper, a series of experiments were carried out under varying conditions of pH, Cu(II) and initial monochloramine concentrations. Results showed that the decomposition rate of monochloramine was greatly enhanced by Cu(II), and this enhancement decreased with the increase of solution pH and the decrease of Cu(II) concentration. It was proposed that the monochloramine decomposition in the presence of Cu(II) was catalyzed via complexation between Cu(II) and monochloramine. The X-ray absorption fine structure experiments gave further evidence to this conclusion. The results will provide useful information for selecting proper disinfection method in water disinfection where Cu(II) exists and reasonable monochloramine dosage during chloramination.