Cellulose, hemicellulose and lignin are generally recognized as the main components of sawdust. In this paper, the spontaneous combustion mechanism of sawdust was studied from the aspect of these components. The thermogravimetry analysis and biochemical component analysis showed that the lignin decomposed evidently at the induction period of thermal decomposition of sawdust, and the lignin contents of sawdust also presented positive correlation with their spontaneous combustion risk. Scanning electron microscope, fourier transform infrared spectroscopy and X-ray photoemission spectra were implemented further. The results showed that: the cellulose remained stable at the induction period; for hemicellulose, the oxidation reaction took place, but it was very weak. However, for lignin, the dehydration reaction (loss hydroxyl group of alcohols) took place evidently so that the lignin exhibited an apparent porous structure. Therefore, the dehydration reaction of lignin might greatly affect the spontaneous combustion of sawdust.