Abstract: A new resonance light quenching method for the quantification of trace selenium has been developed. The method is based on the reaction of Se (Ⅳ) with excess I^- in 0.14 mol/L H₂SO₄ solution to produce I^-₃ with arabic gum (AG). After adding NaAc, I^-₃ reacts with methylene blue (MB) to form an ion-association complex, resulting in resonance light quenching at a wavelength of 288 nm. The optimum concentrations of KI, MB, NaAc and solubilizer AG were decided. In addition, reaction for 15 min at room temperature (20-30℃) was found out. As a result, the sensitivity and stability of the system was improved. Good linear relationship between resonance light quenching intensity and Se(Ⅳ) concentration occurs in the content range of 0-0.24 μg/mL. The detection limit is 1.01 μg/L. The interferences of Fe³⁺, Pb²⁺, Cu²⁺ and Zn²⁺ are eliminated by EDTA. This method was applied to the quantification of trace selenium in environmental water samples with RSD of 1.3%-3.6% (n=5) and recoveries from 97.4% to 100.8%.