In this paper, aiming at the phenomenon of abnormal increase of CO concentration in fully mechanized caving face of No.15 coal seam in Shanxi Yangquan Chengzhuang Coal Mine, taking 15305 working face of the mine as an example, borehole sampling was carried out at different positions of the return air trough of the working face, and the original CO content of the coal seam was measured by atmospheric desorption method, and the source of CO in the coal seam was analyzed. Through analysis and calculation, the CO content of coal seam in the two measurement sites of 15305 working face return air crossheading is 1.30×10-4 m3/t and 1.77 ×10-4 m3/t, respectively, of which the resolvable amount accounts for a small proportion. Considering the experimental conditions and drilling cuttings heating and other factors, it is considered that there is no primary CO in No.15 coal seam of Chengzhuang Coal Mine, which provides data support for the formulation of mine fire prevention and extinguishing measures.