In this study, we performed density functional theory (DFT) calculations to investigate the role of Cd single-atom catalyst on the nitrogen-doped carbon (Cd-SA/NC) as the cathode material for Li-CO2 batteries. Employing a model of Cd-SA/NC combined with carbon vacancies and doped nitrogen atoms in graphene, we first calculated the binding energy of the catalyst to evaluate the thermodynamic stability. Then, we revealed that the activation energy required for the decomposition of Li2CO3, which is the charge reaction in Li-CO2 batteries, was lower for the Cd-SA/NC than the bare nitrogen-doped carbon. Furthermore, it was shown that the Cd-SA/NC had a lower thermodynamic barrier than the nitrogen-doped carbon in the formation of Li2CO3, which is the discharge reaction of Li-CO2 batteries. Overall, we demonstrated the high activity of Cd-SA/NC as a cathode catalyst of a Li-CO2 battery.