A benzoquinolizine coumarin-based fluorescent probe was developed for detecting thiophenols, demonstrating the superior fluorescence properties caused by the decay of the twisting effect of N,N-diethylamino group of coumarin. It discriminated thiophenols from various analytes including aliphatic thiols with good selectivity and displayed ∼700-fold fluorescence intensity enhancement and a remarkable limit of detection (4.5 nM). The new probe also can be applied to quantitative determine the concentrations of thiophenol in water samples and living cells.