Abstract: To effectively resolve the problem that CO gas concentration of Tianchen Coal Mine internal Ⅲ lower 518 working face is too high, the source of CO gas was found at first, and then the reasons of harmful gas influx were analyzed. The main reasons for the influx of CO gas from mine goaf to working face were coal destruction caused by the advanced support pressure, expansion of coal pillar fracture caused by caving in goaf, many small faults in the vicinity of internal Ⅲ lower 518 working face. In these regard, pressure balancing ventilation, gas drainage, coal seam water injection and grouting plugging were used. After detection of the working surface, the result that the comprehensive utilization of these measures can effectively solve the problem of high concentration of harmful gas in internal Ⅲ lower 518 working face can be obtained.