To explore the reaction mechanism of the low-temperature oxidation of coal and the law of active groups producing CO through oxygenolysis, this article analyzed the reaction characteristics of free radicals and functional groups during low-temperature oxidation by using electron spin resonance and Fourier transform infrared technologies. Based on gas chromatograph analysis, the index gas CO produced by active groups during coal spontaneous combustion was analyzed by studying the reaction of free radicals and oxygen-containing functional groups. The experiments showed that, with rising oxidizing temperatures from 30 degrees C to 230 degrees C, the g-value increased first and then decreased, while the concentration of free radicals constantly rose by 48.3%. When the oxidation temperatures rose to 100 degrees C, the maximum g-value occurred, and the concentration of free radicals changed from slowly increasing by 10.2% to dramatically rising by 31.7%. Moreover, the relative intensities of various oxygen-containing functional groups, including -OH, C=O, C-O, and -COOH, exhibited different change laws with rising temperatures. The -OH constantly declined by 67.0% and began to slowly decrease when the temperatures reached 100 degrees C, while C=O declined first, then increased, and began a rapidly rising stage after the temperatures increased to 80 degrees C. The C-O was nearly unchanged below 180 degrees C, while the growth was accelerated after reaching 180 degrees C, and -COOH decreased at first, then rapidly increased after 80 degrees C. Based on the free radical theory of coal spontaneous combustion, it was revealed that carbonyl radicals are the important active groups to produce CO. It can be assumed that a C=O functional group can also produce CO by taking phenylacetaldehyde as an example. A C-O functional group can produce CO by generating carbonyl radicals, so carbonyl radicals are the direct active groups to produce CO during low-temperature oxidation. CO concentration sharply increased after the oxidizing temperatures reached 100 degrees C, which is consistent with the change of concentration of free radicals. There is a significant inflection point of oxygen-containing functional groups at 80 degrees C, so 80-100 degrees C (especially 100 degrees C) is the allowable maximum temperature of the low-temperature oxidation of coal mass.