Chemical fixation of CO2 into useful organic compounds has attracted much attention in both academia and industry from the standpoint of environmental protection and resource utilization. In this respect, the fixation of CO2 with epoxides catalyzed by quaternary onium salts has been successfully achieved and commercialized, however, the details of the mechanism are still unclear. In this work, we investigated the mechanism of fixation of CO2 with ethylene oxide catalyzed by quaternary ammonium salts by experimental and density functional theory (DFT) studies. Through the study, the detailed structural and energetic information about each step of the catalytic cycle were obtained. In addition, several factors, such as the chain length and anion, affected the reaction mechanisms, and the outcomes were also identified by experimental and DFT studies. This work provides a molecular level understanding of the reaction process and forms the basis of rational design of catalytic system.