The photochemical persistence of chemicals is a key factor to determining their environmental risk. Revealing the key mechanism of environmental phototransformation of chemicals is of great significance to chemical risk management. In this study, phototransformation kinetics and mechanism of bisphenol A diglycidyl ether (BADGE) in coastal seawater was investigated through simulated sunlight irradiation experiments and density functional theory (DFT). Phototransformation plays an important role in the loss of BADGE and rate constants in pure water, artificial seawater and seawater are (2.5 ± 1.0) × 10-4 min-1, (2.5 ± 0.5) × 10-3 min-1 and (2.1 ± 0.4) × 10-3 min-1, respectively. Cl2·- mainly participates in the phototransformation of BADGE by attacking to O atom of epoxy ether after the addition of H+ and leads to the formation of three chlorinated products in seawaters and artificial seawater which was confirmed by DFT calculation and products identification. Acute toxicity prediction to fathead minnow indicates that chlorinated products exhibit higher acute toxicity than BADGE. BADGE·2HCl as one of the final phototransformation products for BADGE could undergo product-to-parent reversion phototransformation back to BADGE and the rate constant is 1 order of magnitude lower than that of BADGE. This study provides an insight into the photo-induced product-to-parent reversion of BADGE and such reactions can also occur in chemicals with three-membered epoxide groups in coastal waters. Comprehensive consideration of phototransformation pathways and toxicity changes between the parent and products is of great significance for the accurate assessment of ecological risk for organic pollutants.