Phosgene is a highly toxic gas that poses a serious threat to public health and safety. Herein, we describe the preparation of a ratiometric fluorescence probe (Pi) bearing hydroxyl and imidazole moieties as recognition sites, and employ it for the excited-state intramolecular proton transfer-based (ESIPT-based) detection of phosgene. It is the first time that hydroxyl and imidazole have been exploited as recognition sites for phosgene. In the presence of phosgene, Pi undergoes sequential nucleophilic substitution and cyclization reactions that facilitate a rapid response, high selectivity, and excellent sensitivity (detection limit = 0.14 μM). The sensing mechanism was verified by ¹H NMR spectroscopy and high-resolution mass spectrometry. Furthermore, we fabricated a fluorescent test strip (FTS-Pi) for the detection of phosgene in the gas phase that undergoes a fluorescence color change, from green to blue, under 365 nm UV light in the presence of phosgene, which is easily observed with the naked eye.