The thermal decomposition of oxygen-containing functional groups produces CO, CO2 and active sites that accelerate coal spontaneous combustion. Study on this process is of great significance for research on the mechanism of coal spontaneous combustion. In this paper, research was made on the dependency relationship between gas production and time under the condition of different coal samples, particle sizes and pyrolysis temperatures. Results reveal that the time-dependent generation rate of active sites follows an exponential compound function P = A + Be-t/k1 + Ce-t/K2 during isothermal pyrolysis of coal. Besides, infrared spectrum experiments were also carried out to study changes in oxygen-containing functional groups of raw coal, pyrolyzed coal and room temperature oxidized coal after pyrolysis. The results prove that when coal reaches a certain temperature, oxygen-containing functional groups will adsorb heat and undergo decomposition to produce active sites and a few gas products. Then, active sites are oxidized, generating oxygen-containing functional groups and releasing both massive heat and gas products. Therefore, the whole reaction is a continuous cyclic process where combined effects of the two reactions cause rapid accumulation of heat, which triggers uncontrollable coal spontaneous combustion. The results from this research will be helpful to inhibit the occurrence of coal spontaneous combustion by reducing the generation and oxidation of active sites. (C) 2019 Elsevier Ltd. All rights reserved.