The Li-air battery is expected to become the next generation of the energy storage system because of its high theoretical energy density of 3500 Wh/kg (based on Li2O2 formation at the cathode). CO2 (∼300 ppm) in the air is regarded as an impurity for cathode reactions, because it can lead to the formation of Li2CO3, which increases the overcharge potentials, decreases energy efficiency, and gives rise to the serious decomposition of battery components. However, the impact of a low concentration of CO2 (<1000 ppm) on cell performance has not been addressed. In this work, we quantitatively characterized and analyzed the impact of a low concentration of CO2 on the electrochemical performance of Li-air batteries to investigate the tolerance of Li-air batteries to CO2. The discharge capacities and cyclability of the batteries with CO2 below 100 ppm are similar to those without CO2. The batteries with 0, 50, and 100 ppm of CO2 delivered 85, 88, and 83 cycles, respectively. At the same time, the critical byproduct Li2CO3 was quantified, and its effect on batteries is analyzed by in situ electrochemical impedance spectroscopy (EIS) with a distribution of relaxation time (DRT) calculation. This study promises a theoretical basis for developing CO2 removal materials and devices for Li-air batteries in the future.