Based on the Wiser Coal Model, the process of coal hydropyrolysis was studied by molecular dynamics simulations with reactive force field (ReaxFF). The gas production and organic sulfur removal were analyzed. The related reaction mechanism and factors were discussed. It turns out that hydropyrolysis, relative to pyrolysis, could increase the gas yield and improve the removal rate of organic sulfur up to 80%. With the rise in temperature or pressure, the desulfurization rate increases first and then decreases. Adding hydrogen could weaken the CAS bonds in thiophene (ring), thiophenol (R-S-H), phenyl sulfide (R-S-R), then the bond-dissociation energy declines and the bonds are easier to break, so the hydropyrolysis could effectively improve the desulfurization rate. The simulated temperature-dependence of desulfurization rate is in agreement with the reported experimental tendency. (C) 2014 Elsevier Ltd. All rights reserved.