Objective To explore the phototransformation of a synthetic progestin, dydrogesterone (DYD), under simulated sunlight, and identify its potential transformation pathways. Method Photoreactor was used to test the phototransformation process of DYD under simulated sunlight. Experiments were also conducted to explore different pH (5, 7, and 9), temperature (15, 25, and 35 ℃), and natural organic matter (NOM) concentration (0, 5, and 20 mg·L^-1) on DYD phototransformation rate. Ultra-high performance liquid chromatography-time of flight mass spectrometer (UHPLC-QTOF-MS) was used to identify the potential photoproducts of DYD, and the potential transformation pathways were speculated. Result The concentration of DYD decreased over time under simulated sunlight, indicating that phototransformation occurred in the system. DYD transformation rate was not obviously affected by the initial solution pH, and the first-order phototransformation kinetic rate constant (k) was 0.015−0.019 h^-1. DYD transformation rate was facilitated by high temperature and low concentration of NOM (k increased from 0.019 h^-1 to 0.027 and 0.028 h^-1, respectively), but was inhibited by high concentration of NOM. UHPLC-QTOF-MS analysis showed that 19 photoproducts were generated during transformation, via hydroxylation, hydrogenation, polycondensation, and photoisomerization with subsequent ring-opening reactions. Conclusion Phototransformation will occur for DYD in surface water, and temperature and NOM play important roles in DYD phototransformation. A total of 19 photoproducts detected in this study remain the basic molecular skeleton of DYD, indicating that these products would exhibit potential endocrine disruption to aquatic systems.