Abstract: Xianfeng lignite (XF) was used as raw material to investigate its structural evolution during hydrothermal treatment in a 500 mL autoclave.The structure was characterized by solid state ¹³C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FT-IR).The results show that the carbon contents increase and oxygen contents decrease after hydrotnermal treatment.The weak bonds in lignite are broken and the oxygen functional groups decrease gradually during hydrothermal treatment below 240℃.Hydrogen is donated from water to lignite through ionic pathway and transferred into lignite.The methyl carbon increases at 200℃ and then decreases.The methenyl carbon increases from 4.80% in XF to 13.16% in XF-240.The released gas is mainly CO₂ during hydrothermal treatment.The hydrocarbon composition in released gas increases with the treated temperature.The covalent bonds are broken above 240℃.The hydrocarbon composition in released gas increases from 2.13% at 240℃ to 8.59% at 300℃.The aliphatic carbon in lignite deceases from 44.83% in XF-240 to 39.49% in XF-300.The oxygen-linked carbon in lignite decreases from 12.57% in XF-240 to 1.49% in XF-300.Hydrothermal treatment plays a role in deoxygenation and upgrading of raw lignite.The oxygen contents decrease by about 30% and the aromatic carbon increases to 60.50% at 300℃.