Air−light hydrocarbon mixing gas is a new type of city gas which is composed of light hydrocarbon with the main component of n-pentane and air mixed in a certain proportion. To explore the dominant reactions for CO production of air−light hydrocarbon mixing gas with different mixing degrees at the critical equivalence ratios, a computational study was conducted on the combustion characteristics, including the ignition delay time, laminar flame speed, extinction residence time, and emission of air−light hydrocarbon mixing gas at atmospheric pressure and room temperature in the present study. The calculated results indicate that the ignition delay time of air−light hydrocarbon mixing gas at temperatures of 1000−1118 K is greater than that of n-pentane, while the opposite at temperatures of 1118−1600 K. From the study of the laminar flame speed and ignition delay time, it was found that the essence of air−light hydrocarbon mixing gas is that its attribute parameter is determined by the ratio of n-pentane to the total amount of air at the moment of combustion. The changes in the extinction residence time and the CO emission index of air−light hydrocarbon mixing gas are not synchronized, that is the CO emission index is not necessarily small for air−light hydrocarbon mixing gas with excellent extinction residence time. CO sensitivity analysis and CO rate of production identified key species and reactions that are primarily responsible for CO formation and annihilation. The mixing degree plays a key role in the CO emission index of air−light hydrocarbon mixing gas, which has a constructive opinion on the future experiment and application of air−light hydrocarbon mixing gas.