To improve CO2 adsorption on surfaces of alkali metal oxides, M2O, where M = Li and Na, two different strategies were examined using first-principles calculations. CO2 adsorption energies on these surfaces were negatively increased by the introduction of dopant atoms larger than host metal atoms of surfaces. Among Li, K, and Rb dopants, Rb was the most effective one in strengthening the adsorption of CO2 on a Na2O(111) surface. To elucidate the charge transfer between CO2 and doped surface, Bader charge analysis was conducted. Moreover, biaxial strain effect on CO2 adsorption on the Rb-doped Na2O(111) surface that showed the negatively highest adsorption energy of CO2 was investigated. The applied tensile strain was helpful in strengthening CO2 adsorption on the surface.