We report an electrochemical device for portable on-site detection of gaseous CH 3 I based on PVIm-F for the first time. The device achieves detection of gaseous CH 3 I with a significant selectivity and a low detection limit (0.474 ppb) in 20 min at 50 °C and 50% relative humidity, which is of great significance for achieving real-time on-site monitoring of radioactive hazardous environments.