In aquatic settings, photodegradation is a significant non-biodegradation method of pesticide degradation. In this investigation, the photodegradation of the herbicide Diquat in aqueous solution was examined under distinct conditions and concentrations. The initial concentration of Diquat and the pH value of the water environment significantly affected the half-life of photodegradation of Diquat, and the opposite was true for different types of water. The photodegradation transformation products (PTPs) formed by the photodegradation of Diquat in an aqueous solution under UV-Vis irradiation were separated and identified using ultra-high-performance liquid chromatography coupled with time-of-fight mass spectrometry (UPLC-QTOF-MS). Based on mass spectral information, four main transition products were identified. Calculations of constituent composition, a comparison of structural analogs, and extant literature all contributed to the structure's determination. The main pathways of photodegradation were found to be oxidation, pyridine ring-opening, cleavage of [C-C] on the piperazine ring, Demethylation reaction and Hydroxylation. These results serve as a foundation for further environmental risk assessment and help explain how Diquat behaves in aquatic environments.