Due to the significant applications in bioimaging, sensing, optoelectronics etc., photoluminescent materials have attracted more and more attention in recent years. 1,5-Benzodiazepin-2-one and its derivatives have been used as fluorogenic probes for the detection of biothiols. However, their photochemical and photophysical properties have remained ambiguous until now. In this work, we have adopted combined static electronic structure calculations and nonadiabatic surface-hopping dynamics simulations to study the photochemical mechanism of 1,5-benzodiazepin-2-one. Firstly, we optimized minima and conical intersections in S₀ and S₁ states; then, we proposed three nonadiabatic decay pathways that efficiently populate the ground state from the Franck–Condon region based on computed electronic structure information and dynamics simulations. In the first pathway, upon photoexcitation to the S₁ state, the system proceeds with an ultrafast excited-state intramolecular proton transfer (ESIPT) process. Then, the molecule tends to rotate around the C–C bond until it encounters keto conical intersections, from which the system can easily decay to the ground state. The other two pathways involve the enol channels in which the S₁ system hops to the ground state via two enol S₁/S₀ conical intersections, respectively. These three energetically allowed S₁ excited-state deactivation pathways are responsible for the decrease of fluorescence quantum yield. The present work will provide detailed mechanistic information of similar systems.