Accelerator mass spectrometry is the only method for measuring ¹²⁹I in low level environmental samples. In this method, it is essential to convert organic associated iodine into inorganic form for the determination of total ¹²⁹I or organic ¹²⁹I because AgI is usually adopted as a target for AMS measurement of ¹²⁹I. The chemical oxidative method to release iodine from organic matter in natural water was investigated using anion exchange chromatography and CHCl₃ extraction methods. K₂S₂O₈ was confirmed to be an ideal oxidative reagent for decomposing organic matters and converting organic iodine to inorganic form. More than 95% of iodine in natural water can be separated by solvent extraction after oxidation under optimal conditions, and the isotopic exchange of iodine in inorganic and organic forms was well completed during the oxidation, being able to result in an identical ¹²⁹I/¹²⁷I ratio as in the original water. Our works indicate that the reported chemical oxidation method is a suitable approach for releasing organic associated iodine in natural water to allow separation of iodine and preparation of the AgI target for AMS measurement.