FactSage6.1 software simulation and experiments had been used to analysis the reaction mechanism and influence factors for CaS generation during the process of phosphogypsum decomposition. Thermodynamic calculation showed that the reaction for CaS generation was very complex and CaS was generated mainly through solid-solid reaction and gas-solid reaction. The proper CO and CO(2) have benefit for improving the decomposition effects of phosphogypsum and reducing the generation of CaS at 1100 degrees C. Using high sulfur concentration coal as reducer, the proper reaction conditions to control the generation of CaS were: the coal particle size was between 60 mesh and 100 mesh, reaction temperature was above 1100 degrees C and the heating rate was 5 degrees C/mm. Experimental and theoretical calculation indicated that the concentration of CaS was only ten percents in the solid product at 1100 degrees C, which is favorable for the further cement producing using solid production. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.