DOPO (9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) is emerging organic phosphorus flame retardant, which has gained greatly attention because of its adverse health effects on aquatic organisms. In this study, UV light-emitting diodes lamp driven sodium sulfite activation (UV₂₅₄/Sulfite) process was employed as an advanced reduction process to remove DOPO, and its mechanisms were revealed. The pseudo first order rate constant of DOPO (k_obs-DOPO) in UV₂₅₄/Sulfite process initially decreased from 6.95 × 10⁻⁵ s⁻¹ to 5.11 × 10⁻⁵ s⁻¹, and subsequently increased to 1.61 × 10⁻⁴ s⁻¹ as the sulfite concentration increased from 0 to 10 mM, indicating that the negative effect of HSO₃^- on DOPO degradation was the dominate factor in UV₂₅₄/Sulfite process at low sulfite concentration (< 2 mM), while the yields of hydrated electron (e_aq^-) increased as the sulfite concentration gradually reaching to a high value (10 mM), resulting in enhancing DOPO degradation significantly. In addition, the coexisted water matrix in deionized water and natural surface water weakened DOPO degradation in UV₂₅₄/Sulfite process, and this inhibition strengthened as the concentrations of water matrix raised. The DOPO degradation pathway indicated that e_aq^- attacked phosphaphenanthrene structure of DOPO and contributed to its detoxification which was confirmed by Quantitative Structure-Activity Relationship (QSAR) assessment.