The mechanism of the CO2 splitting reaction with N-heterocyclic carbene (NHC) as a catalyst and cinnamic aldehydes as the oxygen acceptor was thoroughly investigated at the B3LYP/6-31G(d,p) level. Owing to its nucleophilicity, NHC can initiate the reaction through two distinct channels (I and II) by nucleophilic attack. The results reveal that without the assistance by H2O, channel II is slightly more favorable than channel I, and the reaction path II-D is the optimum reaction path. However, the reaction is difficult to conduct at room temperature because of its high activation energy barrier. Although the results suggest that most of the high activation energy barriers involved in proton transfer decrease if the reaction is assisted by H2O, all highest energy barriers for all reaction channels are still above 40 kcal mol−1. This indicates that this reaction cannot be performed at room temperature, which is also proven by experiments.