Ionic liquids (ILs) have excellent ability to capture CO2 due to their unique properties. The observation on the evolution process of solid-supported IL nanomembranes at the nanoscale benefits understanding the interaction between IL nanomembranes and CO2. In this work, we have investigated the CO2-induced evolution process of solid-supported nanomembranes formed by the imidazolium-based ILs with alkyl chains of different lengths but with a common anion by in-situ atomic force microscopy (AFM). The morphology evolution and mechanical properties of the IL nanomembranes have been quantitatively analyzed. The results show that similar phenomena were observed for the three IL nanomembranes, i.e., the absorption of CO2 presents a maximal impact on the innermost layer of the solid-supported IL nanomembranes. The innermost layer of IL nanomembranes becomes fragmented and tends to re-assemble into a multi-layer structure, while the area of the multi-layer IL nanomembranes expands due to the incorporation of some small regions. In addition, the morphology changes of IL nanomembranes are dependent on the length of cation alkyl side chain. For the [BMI][TFSI] nanomembrane, the morphology change is faster than the other two IL nanomembranes in the preliminary stage of CO2 absorption and reaches a stable state in a shorter time. AFM quantitative nanomechanical measurements show that Young's moduli of the three IL nanomembranes significantly decrease after CO2 absorption, which supports that CO2 molecules could weaken the interaction among IL anions, cations and mica, and thus lead to the morphology changes of the three solid-supported IL nanomembranes. Our work provides microscopic insights into the process of CO2 absorption by ILs, which lays a foundation for further studying the interaction between CO2 and solid-supported IL nanomembranes.