The structure evolution in the process of low rank coal hydrocarbon generation was studied using Fourier transform infrared spectroscopy and gas chromatography. Gaseous hydrocarbon yield and change law of functional groups were obtained. The results show that: the coal pyrolysis products are mainly gaseous hydrocarbon C1-5. Methane generation instantaneous yield curve contains four peak of hydrocarbon pyrolysis. Oxygen-containing functional group and alkyl side chain of low rank coal chemical structure reduced while aromatization degree increased along with coal rank. The characteristic absorption peak of coal structure of aliphatic hydrocarbons, aromatic hydrocarbon, methyl CO base CC of alkanes and aromatic structure of methyl and methylene were characterized by 2 950, 2 920, 2 860, 1 730, 1 705, 1 600 and 1 380～1 460 cm-1 selected in FTIR spectra. Temperature 420 ℃ is the turning point, before the absorption peak intensity gradually decreases, and then increases slightly. Three major structural evolution stages of coalification mechanism were revealed. Finally, the low rank coal hydrocarbon structure evolution pattern was put forward.