The mechanism of cycloaddition reaction between carbon dioxide and epoxide, catalyzed by HEMIMC (1-(2-hydroxyl-ethyl)-3-methylimidazolium chloride), was investigated using the DFT (density functional theory) method. In the presence of HEMIMC, the reaction mechanism changed from single-step to multipath. Seven reaction pathways are reported here, including two steps, epoxide ring-opening and ring-closure of cyclic carbonate, or three main steps, epoxide ring-opening, carbon dioxide insertion, and ring-closure of cyclic carbonate. The catalytic activity of HEMIMC was studied, and the catalytic mechanism was elucidated. The nucleophilic attack of anion and hydrogen bond are two of the most important factors to promote the cycloaddition reaction, especially the OH functional group in HEMIMC. Finally, the influence of different anions on the catalytic activity was investigated.