Hydrothermal treatment of Xiaolongtan lignite (XLT) was carried out in a lab autoclave and the pyrolysis was studied in a tube reactor at 873 K for 15 min. The structure of the lignite was characterized by solid state C-13 nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results show that the lignite structure and pyrolysis product distribution have a significant change after hydrothermal treatment. Hydrothermal treatment is divided into three stages with the temperature increasing. The oxygen functional groups decompose and substituted by hydrogen in the first stage (<493 K). The pyrolysis tar yields increase from 7.97 wt.% of XLT to 9.60 wt.% of XLT-493 and the pyrolysis water decreases. In the second stage (493-533 K), hydrothermal treatment disrupts the weak covalent bond such as ether, ester and side-chain substituent by hydrolysis. Meanwhile, hydrogen transfer takes place between water and lignite or among the intra-molecular structure lead to increment of the CH2/CH3 ratio. The pyrolysis tar yields increase to 9.76 wt.% of XLT-513 and the pyrolysis water decreases sequentially. Then, the pyrolysis water increases and pyrolysis tar decreases because of the obvious increment of the oxygen-linked aromatic carbon and little increase of the ratio of CH2/CH3. The pyrolysis water and tar decrease subsequently in the third stage (>533 K) as the result of the cleavage of covalent bonds. (C) 2015 Elsevier Ltd. All rights reserved.