Conversion of carbon dioxide (CO2) to valuable materials is important to academia and the chemical industry. Fixation of carbon dioxide catalyzed by ionic liquids (ILs) is an attractive option. Much work has been done in the fixation of carbon dioxide catalyzed by ILs, but most of it focused on reactions catalyzed by one molecule. In this work, we studied the existence of IL clusters in a series of systems containing hydroxyl-functionalized ILs (HFILs) through a series of theoretical calculations. The intermolecular interactions and microstructures of IL clusters were studied by molecular dynamics (MD) simulations. The results indicated that three-ion clusters to six-ion clusters exist in systems, while the number of clusters decreased as the cluster size increased. Instead of studying the single ion or ionic pair catalytic processes, we gained more insight into the catalytic effects of ionic liquid clusters on the fixation of CO2 by using density functional theory (DFT) calculations. Results suggested that the activity of ILs could be enhanced to a certain extent in the presence of clusters.