The widespread use of chloramination as a disinfection strategy has raised concerns about the formation of harmful disinfection byproducts (DBPs), highlighting the urgent need to elucidate the underlying transformation mechanisms. In this study, we integrated non-targeted screening, organic synthesis, and quantum chemical calculations to investigate the chloramination mechanism of preservative methylparaben and assess the toxicological impacts of the resulting DBPs. In addition to chlorinated products, two novel transformation products, the coupled (C-MeP) and hydroxylated (OH-MeP) products were identified, and the structure of C-MeP was confirmed through synthesized standards. Quantum chemical calculations reveal that the formation of C-MeP is driven by electron transfer, generating radicals that promote C-C bond coupling, while OH-MeP is likely formed by an addition-hydrolysis pathway. The formation of chlorinated products occurs through electrophilic substitution, facilitated by water molecules, and may undergo coupling to form chlorinated products with a biphenyl structure. Toxicity assessments indicate that coupled products exhibit greater negative effects on the gastrointestinal and cardiovascular system compared to parent pollutant. These findings highlight the need to expand the focus of chloramination disinfection beyond traditional chlorinated DBPs, emphasizing the ecological and health implications of novel products including coupled byproducts.