PME (Phosphate monoesters) are key components of dissolved organic phosphorus, characterized by relatively high abundance and pronounced photoactivity; however, their photochemical dephosphorylation mechanisms in aquatic systems remain to be systematically elucidated. The study systematically investigated the photochemical behavior of the phosphate monoester G6P (Glucose-6-phosphate) by combining experimental investigations with DFT (Density Functional Theory) calculations. The results indicated that the light absorption of G6P is primarily in the 200–290?nm range, with higher photolysis efficiency under shorter-wavelength irradiation. Analysis of energy and electron transfer processes revealed that interactions between G6P and DO (Dissolved Oxygen), DOM (Dissolved Organic Matter), and major anions (e.g., NO3-, HCO3-) do not serve as the main pathways driving its photolysis. Instead, photodegradation relies more on the attack of RIs (Reactive Intermediates) on electron-rich sites, such as the extended region of the glycosyl–phosphate linkage, which triggers molecular destabilization and ultimately leads to IP (Inorganic Phosphate) release. Photolysis experiments further confirmed this mechanism, showing that 3DOM* plays a predominant role in promoting G6P degradation in DOM-containing systems. From an environmental perspective, significant differences were observed in the photolysis of PME between freshwater and seawater systems. The photolysis rate constants of G6P were determined to be (2.00–5.60)?×10-3?h-1 in freshwater and (1.60–3.30)?×10-3?h-1 in seawater. Freshwater systems favor oxidation-dominated pathways with higher thermodynamic driving force, whereas seawater systems, under high salinity and halide-rich conditions, exhibit more pronounced halogen substitution and addition characteristics. Overall, this study demonstrates that the photolysis of PME is co-determined by its molecular structure and ambient aqueous conditions, which collectively govern the pathways and efficiency of its photochemical transformation. These findings provide a new mechanistic perspective for understanding the abiotic mineralization and environmental behavior of DOP (Dissolved Organic Phosphorus) in aquatic systems.