In this paper, it was carried out for reduction decomposition of phosphogypsum to calcium sulfide under N2 atmosphere. The size of phosphogypsum was measured by Nano-ZS90 Zetasizer. Scanning electron microscopy and XRD-ray diffraction were used to analyze the raw-material and decomposition solid production. These conditions were also studied on a carbon to CaSO4 molar ratio, reaction temperature, reaction time, and reaction atmosphere in the N2 atmosphere. The results showed that the optimal conditions for production of CaS are found to be a carbon to CaSO4 molar ratio of 2.4 to 1, a temperature of 900～1000 ℃,a reaction time of 2 h, and reductive atmosphere. Under these conditions the conversion rate of CaS from phosphogypsum amount to no less than 97.60%.