•A rationmetric emission pH probe CzBI is facilely synthesized.•CzBI exhibits a remarkable emission ratio (F605nm/F523nm) enhancement with decreasing pH from 7.00 to 0.50.•The pKa is 2.44 and the linear pH response range lies in the extremely acidic pH range of 1.50–3.60.•CzBI can monitor pH fluctuations in HeLa cells and extremely acidic pH changes (pH 1.0–4.0) in E. coli cells with ratiometric response. A ratiometric emission fluorescent probe 1,1-dimethyl-2-(2-(N-ethyl-carbazol-3-yl)-vinyl)-1H-benzoeindole (CzBI) is facilely synthesized via the ethylene bridging of carbazoaldehyde and benzoindole. The probe exhibits a remarkable emission ratio (F605 nm/F523 nm) enhancement with a pKa value of 2.44, and the linear response over the extremely acidic pH range of 1.50–3.60. CzBI also displays a large Stokes shift of 100nm under extremely acidic conditions, high sensitivity and excellent reversibility, which are favorable for intracellular acidic pH imaging. In addition, the probe has excellent cell membrane permeability and is applied successfully to visualize intracellular pH fluctuations in live cells and, further monitor extremely acidic pH changes in E. coli cells without influence of autofluorescence and native cellular species in biological systems.