A novel pH fluorescent probe imidazo[1,2-a]indole derivative is reported. The probe is highly selective to strong acidic pH (pKa = 3.56) with high sensitivity and a fast response time (within 30 s). It is hardly interfered by ordinary metal ions and has good reversibility under strong acid conditions. The probe transfers charge under different pH conditions, and the response mechanism depends on the change of ICT. It can also be used for imaging in strong acidic Saccharomyces cerevisiae and detection of intracellular H+ as well.