Distributions and conversations of sulfur in hydropyrolysis of lignite greatly influence the quality of its products. In this work, two three-dimensional models of lignite for its pyrolysis and hydropyrolysis processes were built to study influences of H-2 atmosphere on the organic sulfur transforming mechanism. The structural units of the two models are the molecular model provided by Wolfrum. Reactive force field (ReaxFF) molecular dynamics was then used in simulations of desulfurization process for the two models. C++ programs were developed to deal with the ReaxFF results, and analyze the effects of H-2 molecules on the S-atom distributions and S-containing elementary reactions. We found that the temperature should be controlled in an appropriate range in desulfurization process to reach maximum products with minimum S content. The H-2 molecules could help thiophene and thioether structures convert into thiophenol/thiol structures in lignite. Meanwhile, H-2 molecules also help H2S release into the gas category, achieving the desulfurization from the liquid and solid products. This theoretical work will be helpful in understanding the hydropyrolysis process of lignite. (C) 2015 Elsevier B.V. All rights reserved.