Hypochlorous acid (HOCl)/hypochlorite (ClO) was a biologically important component of reactive oxygen species (ROS) and plays a key role in human immune function systems. HOCl/ClO can destroy invasive bacteria and pathogens, and mediate the physiological balance of the organism with low concentrations, and cause oxidation of the biomolecules such as proteins, cholesterol and nucleic acid in biological cells, leading to a series of diseases with over capacity. Therefore, quantifying the content of HOCl/ClO in organisms are extremely urgent. In this work, coumarin-salicylic hydrazide Schiff base (CMSH), a ratiometric and colorimetric fluorescent probe for ClO- detection based on coumarin as the fluorophore unit was rationally designed and synthesized. The results indicated that CMSH exhibits high selectivity and sensitivity for ClO- identification. Additionally, the ratios (I/I) displayed brilliant ClO-dependent quick and sensitive performance within 40 s and limitation of 128 nM, respectively. As well as the color of the solution changes from green to colorless accompanied by the fluorescence form green turns into blue with addition of ClO. Totally, CMSH has been successfully employed as ratiometric sensor to image in living cells, bacteria and zebrafish with low cytotoxicity and good permeability.