Coal gasification technology in supercritical water provides a clean and efficient way to convert coal to H-2. In the present paper, the whole supercritical water (SWC) gasification process of a coal particle is studied with the reactive force field (ReaxFF) molecular dynamics (MD) method for the first time. First, the detailed reaction mechanism which can't be clearly illustrated in experiments, such as the evolution of the carbon structure during the gasification process and the detailed reaction mechanism of the main products, is obtained. According to the generation mechanism of H-2, it is found that the supercritical water gasification process of a coal particle can be divided into two stages with different reaction mechanisms, namely the rapid reaction stage and the stable reaction stage. Then, the effects of temperature and coal concentration in the reaction system on the yield of H-2 are studied. Finally, the transition of N in the coal particle is revealed, in which the precursors of NH3 such as CN, CHN, and CHON are the basic molecular structures for nitrogen atoms during the gasification process at high temperature. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.