In order to accurately grasp the natural combustion process of coal, the thermogravimetric experiments of temperature-programming and coal oxidation process were used to determine the natural ignition index gas and critical value of No. 5 coal seam in Tongfa Mine. The coal seam preferred CO and C2H6 gas as the main indicator gas, and C2H4, C3H8, C2H2 and C2H4/C2H6, C3H8/C2H6 as auxiliary indicators. The concentration of CO and C2H6 appeared as a slow oxidation phase, the concentration of CO and C2H6 gas increased significantly, and the C2H4 appeared in the rapid oxidation stage, C2H2  appeared and entered the intense oxidation stage. The experimental results were applied at 8200 working face. The results showed that the working face was in the slow oxidation stage during the recovery and in the rapid oxidation stage during the retreat.