Abstract: Online monitoring of organic reactions can capture the signal and change of reaction reactants, intermediates and products, so as to deepen the understanding of the reaction mechanism. In this study, extractive electrospray ionization tandem mass spectrometry (EESI-MS) were used to monitor Paal-Knorr about acetonylacetone and aniline under the condition of using methanol as solvent, acetic acid as a catalyst. At different time after the start of the reaction, the molecular ion peak of reactants, intermediates, product and dimer signal were captured, and the changes of them in the course of the reaction with time trends were recorded. This method does not require pretreatment of the analyte, which minimizes contamination of the instrument and greatly reduces the delay time (response time less than 1 s), with the characteristics of high sensitivity, high specificity and fast response. It is able to capture short-lived intermediates, and thereby gives a true reflection of substance changes of chemical reactions. This work provides a new idea for the study of the mechanism of chemical reactions.