Owing to the distinct advantages such as facility, low cost and simplicity, fluorescent chemosensors are highly suitable for field detection and rapid screening of suspected chemical warfare agents (CWAs) such as phosgene. For an effective fluorescent chemosensor, instantaneous response to phosgene is one of the several key considerations in practical applications. In this paper, we reported a fluorescent chemosensor, Phos-3, constructed with o-diamine moiety as the active site, which includes one aliphatic primary amine and one aromatic secondary amine for rapid and selective detection of phosgene via colorimetric and ratiometric fluorescence response. The sensing reaction involved two successive carbamylations between Phos-3 and phosgene. The aliphatic amine of o-diamine possessed stronger nucleophilicity than the aromatic amine and could undergo fast intermolecular (the first) carbamylation with phosgene. The second intramolecular carbamylation was accelerated since the pentacyclic moiety limited the C–N rotation of o-diamine. As a result, the sensing reaction displayed a rapid optical response (∼60 s in solutions) with high sensitivity (LOD = 0.3 nM) and high selectivity. Importantly, Phos-3-embedded polymer nanofibers were prepared by the electrospinning technique for the detection of phosgene in the gas phase. These nanofibers embedded with Phos-3 displayed a quick response (≪1 s) to phosgene with high sensitivity (LOD = 25 ppb) and high selectivity. This study will pave the way for its practical application in rapid screening of phosgene in suspected gas samples.