The solvation dynamics of CO2 in an aqueous solution were investigated using quantum mechanical molecular mechanical molecular dynamics (QM/MM-MD) simulations. It is demonstrated that the formation of H2CO3 occurs through direct reactions between CO2 and nH2O, with extremely high activation barriers in the gas phase. However, in a solution, the activation energy decreases as the number of H2O molecules increases. Specifically, for the CO2 − H2O system, the activation energy is about 32 kcal/mol, while for the CO2 − 2H2O and CO2 − 3H2O systems, it decreases to 28 kcal/mol and 15 kcal/mol, respectively. These findings suggest that the solvation of CO2 in a solution favors a step-wise mechanism.