A colorimetric and fluorescent pH probe 1-(4-methylphenyl)-3-(4-dimethylaminophenyl) acrylketone (MDAK) was facilely prepared. The probe exhibited strong pH-dependent behavior and responded linearly and rapidly to minor pH fluctuations within the range of 1.53–3.96. In addition, MDAK displayed a notably large Stokes shift of 121 nm. MDAK possessed a highly selective response to H⁺ over other metal ions, amion acids and displayed good photostability and excellent reversibility. Furthermore, it can be applied to visualize extreme acidity in E. coli cells.