We developed a merocyanine-based fluorescent probe, NEPB, for tracing hydrazine (N2H4) in a ratiometric manner with large Stokes shifts and long emission wavelength. The fluorescence color of probe NEPB changed from green to yellow upon addition of hydrazine. Probe NEPB displayed high selectivity and sensitivity to hydrazine in solution, and could ratiometrically monitor N2H4 in living cells and zebrafish with low cytotoxicity.