In this work, we report a novel approach which employed substrate-triggered intramolecular addition-cyclization cascade to develop a highly selective fluorescent probe E)-3-(4-(1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-1-(2-hydroxylphenyl) prop-2-en-1-one (P-OH) for NH2NH2. The new sensing mechanism of P-OH for NH2NH2 was investigated in detail by fluorescence spectroscopy, 1H NMR titration, mass spectrometry and control experiments. The synthesized probe showed ratiometric fluorescent response to NH2NH2 with naked-eye color changes from yellow to colorless. It's noteworthy that this probe displayed high sensitivity and selectivity to NH2NH2 over other species, including primary amines, Cys, Hcy, GSH, HS- and HSO3-. Furthermore, the probe can not only detect NH2NH2 in real water samples but also image NH2NH2 in living cells, indicating its potential utility for NH2NH2 sensing in environmental and biological samples.