Over 90% chemical processes involve catalysis. Acquiring the dynamic behavior of catalysts under reaction is the key to understand the catalytic mechanism and then design novel catalysts with high performance. Although indirect information can be obtained through various methods, to directly “see” how the catalysts evolve during reaction at the atomic scale and further reveal the catalytic mechanism remains challenging. To address this issue, we manage to visualize the reaction via environmental TEM with gases into the TEM system and investigate the microstructure evolution of catalyst and its impact on the catalytic performance at the atomic level during real reactions, aiming to provide first-hand data and novel strategy for the design and rational synthesis of catalysts with high performance. In this talk, I will summarize recent progresses in our group, mainly focusing on the dynamic evolution of the active sites of catalysts under reactive environments at the atomic scale. In the last, I will give a brief perspective regarding the challenge and opportunity in this field.