Visualizing a chemical reaction process is critical for understanding the mechanism of the reaction. For example, information on chemical reactions involving single nanocatalysts has significant implications for mechanism research and is vital for guiding the selection of the most active nanocatalysts. In this work, dark field microscopy (DFM) is utilized to observe the electrocatalytic reaction process of Au-Pt core-shell nanoparticles (AuNPs@Pt) as an example. Hydrogen ions were reduced to hydrogen (H2) on the surface of AuNPs@Pt under a certain potential, forming H2 nanobubbles covering the surface of AuNPs@Pt. As a result, the scattering intensity of the nanomaterial was observed to significantly increase under DFM. Therefore, the electrocatalytic reaction process could be monitored in real time by simply observing the scattering intensity change via DFM. Our investigation reveals a different nanobubble evolution process with an average nucleation time and lifetime of 0.69 and 32.34 s, respectively. Moreover, the catalytic activity between different nanomaterials was studied. The relationship between the Pt shell thickness and the average scattering intensity change reveals that the electrocatalytic activity is closely related to the Pt content. Finally, from the brightness of the scattering spot observed by DFM, the temporal and spatial distribution information on the catalytic activity could also be obtained, which is more abundant than the information obtained using the traditional electrochemical method.