A density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been used to study the sensing mechanism of an ON-OFF-ON type fluoride anion fluorescent chemosensor (Bis[[7-(diethylamino)-2-oxo-2H-chromene]methyl-ene]‑carbonothioic dihydrazide (CTC). The current theoretical calculation presents a different sensing mechanism from the experimentally proposed one (Sensor and Actuators B 2016, 222, 823-828). Instead of the combination of CTC deprotonation and poorly emissive excited state tautomer or ICT mechanism, the theoretical results predict the sensing mechanism based on dissociation reaction and excited-state proton transfer (ESPT). The calculated vertical excitation energies both in the ground states and first excited states of different forms of CTC, as well as the potential-energy curves, have completely reproduced the experimental results, providing powerful evidence for our proposed CTC sensing mechanism for fluoride anion.