Abstract: Moisture is one of the important factors affecting coal spontaneous combustion. Coal with different moisture contents was prepared to obtain oxidized coal samples at different temperatures through temperature programming. In situ Fourier Transform Spectrometer was used to determine content of active functional groups in different oxidized coal samples, and to study influence of moisture form on formation and transformation of microscopic functional groups during coal spontaneous combustion. Nitrogen adsorber was used to study specific surface area of different oxidized coal samples and influence of form moisture on changes of micro-pore structure during coal spontaneous combustion. Combined with the change law of functional groups and specific surface area, microscopic action of different form moisture on coal spontaneous combustion process was discussed. The action regime of outside moisture varies with the development of coal spontaneous combustion. Different forms of moisture can promote formation of phenols, alcohols, hydroxyl groups, and carbonyl-containing compounds. Effects of moisture form and its content on change of micro-specific surface area, and on evolution of aliphatic C−H components, hydroxyl-containing compounds and carbonyl-containing compounds during spontaneous combustion of coal are different. While effect of moisture form and its content on formation and transformation of hydroxyl-containing compounds and carboxylic acid compounds are similar.