Abstract: The rapid growth of the nuclear industry poses a great challenge for radioactive waste treatment. Volatile radioactive isotopes generated from uranium fission, especially gaseous iodine, attracted great attention due to their environment and health hazards. The widespread species of radioactive iodine are gaseous elemental iodine (I2) and methyl iodide (CH3I). It is difficult to trap the methyl iodide in high humid condition. Activated carbon and derivative materials are widely used as adsorbents to remove volatile iodine. Triethylenediamine (TEDA) is proposed as impregnation via isotope exchange mechanism. In this study, activated carbon fiber (ACF) with high surface area, heat resistant and chemically stable was used as substrate, TEDA-modified viscose-based activated carbon fiber was prepared by hydrothermal reaction. For fiber hydrothermal modified with 5% (mass fraction) TEDA, the surface area and pore volume decrease about 20%. Compared with conventional impregnation, more TEDA impregnation enters into the microporous structure of activated carbon fibers under hydrothermal environment. The gaseous iodine adsorption properties of modified activated carbon fiber were investigated. Static adsorption experiments of elemental iodine and methyl iodide were performed. The adsorption capacity of elemental iodine and methyl iodide increases by 36% and 50% respectively. Methyl iodide adsorption capacity of hydrothermal modified-ACF is 400 mg/g. The dynamic adsorption experiments show that the adsorption performance of modified activated carbon fiber for methyl iodine decreases with the increase of temperature and humidity. The adsorption performance of activated carbon fiber for methyl iodine is greatly affected by environmental humidity. Breakthrough experiment with 5 ppm of gaseous methyl iodide was conducted under high humid conditions (30 ℃, RH 95%), the feed gas flow rate was fixed at 15 L/min for 12 h. Breakthrough curve indicates that hydrothermal modified-ACF has good adsorption performance with respect to methyl iodide. The radioactive CH3131I adsorption experiments under severe conditions were carried out. The removal efficiency of hydrothermal modified-ACF increases from 29.11% to 94.85%. The affinity of methyl iodide is improved by TEDA-modified. The hydrothermal modified-ACF exhibits the high adsorption efficiency of gaseous iodine.