The complexation of copper ions (Cu(II)) with coexisting organic ligands (e.g. polyethyleneimine (PEI)) in water may pose a new threat to water quality. This study revealed that the maxima concentrations of HNMs, DCAN, and DCAcAm from Cu(II)-PEI complex during UV/chlorine disinfection were 1.54, 2.00, and 1.25 times higher than those from PEI, and the maxima concentrations of HNMs, DCAN, and DCAcAm from Cu(II)-PEI complex during chlorination disinfection were 1.22, 1.31, and 1.10 times higher than those from PEI. Meanwhile, specific Cu(II) concentration (0.25 ∼ 3.0 mg L−1) and alkaline pH were more favorable for forming HNMs, DCAN, and DCAcAm during UV/chlorine disinfection, which were highly dependent on the involvement of HO· and Cl·. Cu(II)-PEI, an in-situ formed stable complex that permitted an efficient interconversion cycle of Cu(II)/Cu(I) via concurrent metal-to-ligand charge transfer and ligand-to-metal charge transfer. The analysis and comparison of extensive products provided solid evidence for the primary role of Cu(I) and the enhanced production of Cl· in increasing HNMs, DCAN, and DCAcAm productions, rather than only Cu(II) catalysis similar to that during chlorination. The new findings broaden the knowledge of the influence of in-situ formed Cu(II)-organic complexes on forming HNMs, DCAN, and DCAcAm during UV/chlorine disinfection.