A fluorescent probe,rhodamine 6G hydrazide for hypochlorous acid（HOCl） was facilely synthesized by a one-step reaction，based on the rhodamine structure as the fluorophore and hydrazine as the recognition group.Its structure was characterized by nuclear magnetic resonance（NMR） and mass spectrometry（MS），and its fluorescence properties of HOCl detection were also investigated.The results indicated that the probe displayed a good sensitivity and selectivity to HOCl.Using an irreversible oxidation-reduction reaction，the probe showed a rapid fluorescence enhancement proportional to the amount of HOCl in neutral buffer with a linear range of 2-400 μmol/L and a detection limit of 0.06 μmol/L，and other common reactive oxygen species（ROS） revealed relatively inactive interfences to the detection system.The probe was successfully applied in the determination of HOCl in tap water and fluorescence imaging of HOCl in living HeLa cells.