Abstract: In order to explore the reaction mechanism of active groups in the low temperature oxidation process of spontaneous combustion coal from micro level, based on low temperature oxidation simulation experiment platform and electron spin resonance (ESR) technology, this paper analyzed the reaction characteristics of free radicals in low-temperature oxidation of coal. Experiments showed that with the increase of coal oxidation temperature, free radical types increased then decreased and concentration continually increased by 48.3%. When the oxidation temperature reached to 100 ℃, there were the most types of free radicals and their concentration increased from a slow increase of 10.2% to a sharp increase of 31.7%. Free radical concentration and types have an inflection point in the 100 ℃, so 100 ℃ is the allowable maximum temperature of low temperature oxidation of coal. When the partial or overall temperature of coal reached to 100 ℃, the effective measures must be taken to prevent the coal from heating up in time. It is very important to predict spontaneous combustion and prevent the fire extinguishing in the process of mining deep coal seam and residual coal spontaneous combustion control in the gob.