In this work, compound 1 has been rationally designed and synthesized as a new fluorescent probe for biological thiols. Notably, probe 1 has almost no background fluorescence (Φf < 0.0001) in aqueous solutions; however, it exhibited fluorescence turn-on response to thiols with high sensitivity (a 246-fold fluorescence enhancement and a low detection limit of 0.22 μM for Cys). Moreover, probe 1 showed excellent thiol specificity over other biologically relevant species. The kinetic studies indicated that the probe responded to thiols rapidly, and the pseudo-first-order rate constants of probe 1 reaction with Cys, Hcy, and GSH were determined to be 1.85842, 0.67656, and 0.51519 min(-1), respectively. A possible detection mechanism was proposed to involve the Michael addition of the thiol to the α,β-unsaturated ketone, followed by a cleavage of the hemiketal group, thereby leading to the formation of a fluorescent 7-hydroxyl coumarin derivative. Furthermore, the optical responses of probe 1 to thiols were studied by TD-DFT calculations. Finally, probe 1 has been successfully applied to the detection of biological thiols in human blood serum. And the intracellular imaging applications established that probe 1 can be used to detect different concentrations of intracellular thiols in living cells.