Understanding the dynamics of chemical reactions at the oil/water interface is fundamentally important but remains experimentally challenging. In this study, we reported an experimental methodology to monitor the time-evolving interfacial tensions during the fast microdroplet generation, enabling probing interfacial reaction processes at the early stage. It was achieved by real-time detecting the dynamic interfacial tension using an automated microfluidic platform with online capillary pressure detection and accurate recognition for the geometric features of generating microdroplets. The reaction between sodium hydroxide and oleic acid with the high concentration ratios was used as the model system. Time-evolving interfacial tensions were measured within a sub-second time scale with various reactant concentrations and two-phase flow rates. The interfacial concentrations of reaction product, sodium oleate, were quantified according to the Frumkin adsorption isotherm model, and the reaction rates were determined to range from 1.2x10-6 to 1.1x10-4 mol/(s·m2), decreasing as the droplet generated. These data further revealed two rate-control mechanisms governing the reaction processes. One was the adsorption-controlled mechanism and the adsorption rate of oleic acid was determined to be 64.7 ± 8.2 m3/(mol∙s). Another was the mass transfer-controlled mechanism and the thickness of the mass transfer boundary layer inside the droplet was determined to range from 4.3 to 84.6 μm. This study developed a microfluidic methodology to probe the interfacial reaction during the fast microdroplet generation, providing quantitative insights into the impacts of reactant transport and adsorption processes on the early-stage reaction rate. The quantitative understanding of the early-stage interfacial reactions can advance controlled and predictable microdroplet reaction technologies for practical applications.