A new pH fluorescent probe 3-(N-butylcarbazol-3-dicyanovinyl) pyridine (BILE), which has a carbazole structure attached to pyridine, is synthesized for extremely acidic conditions. BILE displays excellent pH-dependent performance with a pKa of 2.93 and responds linearly to minor pH changes in the range of 2.60–3.30, which can be used to quantitatively detect pH value based on regression equation, F = 2495.42 × pH – 6316.28. Moreover, BILE has good selectivity, large Stokes shift (122 nm), sensitivity and short response time. More importantly, BILE has been applied successfully to imaging extreme acidity in E. coli cells, indicating that the probe can afford practical application in biological systems.