Ignition energy is of much significance for safety assessment of coal dust explosions. The aim of this study is to investigate the ignition behavior and mechanism of coal dusts. Four types of coal dusts were chosen to measure the minimum ignition energy (MIE) in Hartmann apparatus; and it could be found that the MIE of coal dusts increased with the decrease in the volatile matter content. Meanwhile, the variation laws of functional groups were also investigated by in situ series diffuse reflection FTIR (in situ DRFTIR), and the range of temperature measurements of functional groups were from 30 to 400 degrees C. Results indicated that the main functional groups were aromatic hydrocarbons, -CH3/-CH2- and -OH, respectively. Combining the variation laws of main functional groups and the ignition activity, it was found that in situ DRFTIR made it possible to explain the mechanism of coal ignition. With greater proportions of -CH3/-CH2- and -OH and less proportions of aromatic hydrocarbons, the MIE of coal dust clouds decreased. It demonstrated that -CH3/-CH2- and -OH played a vital role in the coal ignition.