Under realistic operation conditions, CO2 in ambient air causes performance degradation of solid oxide fuel cells cathodes. Here, the effect of CO2 on the performance and stability of La0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ (LSCFN) cathode material under different conditions has been studied, and the degradation of LSCFN caused by CO2 has been explained in detail. According to X-ray diffraction analysis of LSCFN powders, impurity is hardly observed after being annealed in air with CO2. In addition, it is found and explained that the electrical conductivity of LSCFN is not sensitive to the CO2 concentration in air, but the polarization resistance of LSCFN electrode severely increases with the increasing of CO2 concentration. Being annealed under CO2 exposure for a long-term period, LSCFN performs higher CO2 tolerance at 850°C than at 650°C. However, CO2 hardly changes the morphology of LSCFN electrode during a relative long-term operation. The degradation of LSCFN mostly results from competitive adsorption/desorption between CO2 and O2.