Isocyanates is a widely-used chemical in many manufacturing industries, such as polymer industry, pharmaceutical production and production of a variety of agricultural chemicals. However, it is harmful to human health due to the volatility. Therefore, it is necessary to develop methods to detect isocyanates quickly and conveniently, especially to gaseous isocyanates. In this work, a novel fluorescent probe, N-buty-4-hydroxy-1,8naphthalimide, was developed for detection of isocyanates. This fluorescence probe can be synthesized by a simple three-steps synthetic route, and the overall yield of the whole synthesizing process reached at 75%. In the absence of isocyanate, the probe solution displays an emission centering at 596 nm when excited at 370nm, which is yellow to the naked eye. Once isocyanate is added, the fluorescence of solution changes from yellow to blue, and the process finishes in 4 min. The detecting limit of this probe to isocyanates is calculated to be 112nM. It is also proved that this probe possesses excellent selectivity for isocyanate and distinct anti-interference to common organic volatilize compounds. In addition, the reaction mechanism between the probe and isocyanate were proved by HPLC, NMR and ESI-MASS. Results show that the hydroxy group on the 4th position of naphthalene ring of probe reacts with isocyanate group (-NCO) of isocyanate, and resulting in carbamates, which alter 4th substituent group of probe molecule and lead to change of fluorescence. In order to detect the gaseous isocyanates directly, test paper are developed based on N-buty-4-hydroxy-1,8naphthalimide. When the test paper exposed to isocyanates vapor, the yellow fluorescence fade away gradually and a blue fluorescence appear in 6 min. and the test paper possesses excellent selectivity for gaseous isocyanate and distinct anti-interference to common VOCs. In conclusion, This strategy is an efficient way to detect gaseous isocyanates, and it may provide a referable approach for directly monitoring the volatile organic compounds in air.