Abstract: Electrospray mass spectrometry (ESI-MS) has a wide range of applications in reaction monitoring due to its high sensitivity, fast response, high resolution, and in-situ online monitoring without sample pretreatment. Currently, the application of electrospray ionization mass spectrometry for reaction monitoring mainly includes offline and online monitoring modes. For the offline monitoring by the traditional mass spectrometry, the reactions still can not be examined in real-time, which challenge the capture and characterization of short-lived intermediates in situ. To make a better in-situ and real-time monitoring of reactions, a series of electrospray-based ambient mass spectrometry techniques have been developed based on the design and improvement of traditional devices. Therefore, the online monitoring of dynamic changes of reactants, intermediates and products has been achieved in-situ and in real time, which is beneficial to mechanism studies and the further improvement of reactions. This article reviewed the recent developments on reaction monitoring and mechanism research by electrospray mass spectrometry. It mainly focused on online electrospray mass spectrometry technology, containing desorption electrospray ionization mass spectrometry (DESI-MS), extraction electrospray ionization mass spectrometry (EESI-MS), nanoelectrospray ionization mass spectrometry (nESI-MS), sonic spray ionization mass spectrometry (SSI-MS) and other online techniques used in the reaction monitoring. Finally, the development trend of electrospray mass spectrometry in reaction monitoring was summarized and prospected. As reviewed, electrospray-based ambient mass spectrometry has shown broad applications in high-throughput screening, in-situ monitoring and accelerated reactions. In the future, accurate quantitative and studies on complex reaction systems need to be further improved during the reaction monitoring.