The adsorption properties of CO2 molecule on the LaCoO3 (110) surface with one/two O2 pre-adsorption are studied by density functional theory method. Our calculated results show that the CO2 molecule acts as an acceptor when it is introduced into the LaCoO3 (110) surface with one O2 pre-adsorption. It is inconsistent with the experimental results. It cannot explain the reason why the resistance of p-type perovskite oxide material increase after CO2 molecule is introduced. For the LaCoO3 (110) surface with two O2 pre-adsorption, the role of CO2 molecule transforms to donor from acceptor. In this mode, the electrons of CO2 molecule transfer to the LaCoO3 (110) surface with two O2 pre-adsorption, resulting in a decrease of hole–carrier concentration and an increase of resistance. The interaction between CO2 and the LaCoO3 (110) surface with two O2 pre-adsorption is responsible for the CO2 sensing characteristics and mechanism of LaCoO3.