The investigation of reaction kinetics is the key to understanding the nature of reaction processes. However, monitoring fast photochemical reactions by mass spectrometry remains challenging. Herein, we developed an optical focusing inductive electrospray (OF-iESI) mass spectrometry platform for real-time and in-situ photoreaction monitoring. Coaxial irradiation from back of nanoelectrospray emitter with a taper section was utilized, so the emitter could act as optical lens to help achieving much larger optical power density at emitter tip compared to other sections, which allowed for in-situ reaction monitoring of photoreactions. Through theoretical calculations, the highest optical power density region volume was ca. 45 nL. We also integrated a controller for the laser source (450 nm), enabling the modulation of pulse du- ration ( > 1 ms). This facilitates the study of photochemical reaction kinetics. The in-situ capability of this device was proved by capturing the short-lived photogenerated intermediates during the dehydrogenation of tetrahydroquinoline. This device was further used to investigate the kinetics of triplet energy transfer based Paternò–Büchi reaction. The reaction order has hitherto remained undetermined while the result of OF-iESI suggested it followed pseudo -second-order reaction kinetics. The short-lived donor-acceptor collision complex intermediate was also successfully identified by tandem mass spectrometry.（DOI：10.1016/j.cclet.2024.110708）