Based on coumarin as fluorophore, 4-bromobutyryl as recognition group, a ratiometric fluorescent probe COCB for hydrazine was designed and synthesized. Its structure was confirmed by 1H NMR, 13C NMR and HRMS. The selective deprotection of the 4-bromobutyryl of COCB by hydrazine led to the recovery of the intramolecular charge transfer (ICT) process. COCB exhibited an attenuation of blue emission at 420 nm and an enhancement of cyan emission at 480 nm, indicating that it can be used as a ratiometric chemosensor for hydrazine. The probe COCB showed high selectivity, high sensitivity and strong anti-interference ability for hydrazine, and could detect hydrazine in a wide linear range (0-250 μmol/L) and pH range (6-11) with a detection limit as low as 0.15 μmol/L. In addition, COCB was easy to synthesize and had low cytotoxicity, and had been successfully used for the detection of hydrazine in actual water samples, soils and living cells.