Two structural models for pyrolysis and hydrothermal treatment of lignite were constructed to investigate the impact of water at high temperature on the structure and reaction processes of lignite. The chemical model proposed by Wolfrum was used as the structural unit of the two models. Reactive force field molecular dynamics was employed to simulate the reaction process of the two models at 1000-3000 K over a period of 300 ps. The characteristics observed in the simulation agree well with the known characteristics of the lignite structure and reactions. The effects of temperature on the product distributions, intermolecular interactions and elementary reactions were analyzed. We found that the temperature should be controlled in an appropriate range in hydrothermal treatment to reach maximum ratio when extracted by organic solvents. The added water molecules can form hydrogen bond with O-containing groups and complexes with metal atoms, destroying intermolecular interactions in lignite. The hydrogen radicals from water molecules can inhibit cross-linking reactions between relatively small fragments, thereby decreasing molecular weights distribution of lignite. Furthermore, water molecules are also favorable to releasing O-containing groups and metal atoms at high temperatures. This work is an intensive study on hydrothermal-treatment mechanism at high temperatures at the atomic level. These conclusions could be helpful for the clean coal technology of lignite. (C) 2015 Elsevier Ltd. All rights reserved.