Abstract: The high content of temperature and toxic and harmful gas in the coal field fire area seriously threatens the safe mining of the working face of coal mining in the lower well. In this paper, according to the mechanism of liquid nitrogen fire-fighting, the injection mode of liquid nitrogen is analyzed, the injection quantity and other parameters are calculated theoretically, and the mathematical model of influencing radius r is established based on the theory of random medium of rock strata movement, so as to determine the reasonable position of liquid nitrogen injection. Using the above methods, the method is applied in Wuhushan Mining Company, and the results show that the temperature and CO concentration of 10^# fire area decreased rapidly after injecting liquid nitrogen into the fire area of 10^# coal field that affects mine safety, and the combustion state of 10^# fire has been effectively controlled, and nitrogen inerting isolation belt is formed in 7^# coal seam goaf between 10^# fire area and the working face, which effectively ensure the safety of the working face in mining.