Abstract: At present, many studies have come to the conclusion that the coal seam contains primary CO gas, but the possibility of CO being adsorbed by coal after CO generated in drilling construction is not considered. In order to explore whether there is primary CO in spontaneous combustion coal seam in Northwest China, the original coal seam in-situ drilling detection method is used to detect primary CO. Three test boreholes are arranged in a row along the roadway side in the solid coal area not affected by mining. After the boreholes are sealed, high-purity N₂ is used to replace the gas in the closed gas chamber, and the gas in the boreholes is extracted by a special air pump, so as to eliminate the impact of CO generated by coal oxidation on the test results during the construction of in-situ detection boreholes. On the basis of analyzing the source possibility of primary CO in coal seam and its emission theory, the variation characteristics of gas concentration in closed borehole with time are discussed. The results show that volume fraction of O₂ and CO in the sealed borehole decrease rapidly with the extension of sealing time, and the volume fraction of O₂ is stable below 2% after 12 days. After 12 days, the CO volume fraction is lower than 10⁻¹², and no CO gas is detected by gas chromatograp. The gas in the borehole is mainly N₂. It is concluded that there is no primary CO gas in the tested coal seam. The results of coal breaking test in N₂ environment and coal sample oxidation test at normal temperature and constant temperature show that CO gas detected at the initial stage of borehole sealing comes from coal breaking operation in drilling construction.