Abstract: Identifying the source of CO and finding out the reasons of abnormal CO emission and concentration overrun can guide coal mine safety production. Taking 6103, 6104 and 6106 coal mining faces of Chuancaogedan Mine in Inner Mongolia as the research object, this paper analyzes the variation characteristics of CO concentration in working face and closed goaf, determines the main and secondary sources of underground CO, and discusses the influence of geological and engineering factors on CO concentration overrun. The results show that there is a good negative correlation between CO and O2 concentration in statistical gas samples. CO concentration overrun mainly occurs in the upper corner of mining face, supports, closed goaf and other places with poor ventilation conditions. CO mainly comes from coal mining oxidation spontaneous combustion and there is secondary CO in coal seam naturally oxidized locally. The geological reasons of CO concentration overrun are shallow buried coal seam, development of reverse fault and low coal metamorphism degree; the engineering reasons of CO concentration overrun are too long working face length and unreasonable ventilation mode. Under the synergetic action of geological and engineering factors, the mining oxidized CO and natural oxidized CO emission superimposed, andaccumulated in the position of low air flow velocity, resulting in underground CO concentration exceeding the limit.