Phosphogypsum decomposition is very complex because of its complicated compounds. In this study, using high-sulfur-concentration coal as a reducer, thermogravimetric analysis, differential thermal analysis, scanning electron microscopy, and X-ray diffraction were used to characterize the decomposition process of phosphogypsum in a nitrogen atmosphere at different conditions. Mechanism analysis and experiment results showed that the amount of the intermediate production of CaS depended on the heating rate and the size of coal used, which was produced mainly through the reaction between CaSO4 and C, and production would decrease with an increase of the coal size. 60 mesh was a suitable size of coal for phosphogypsum decomposition to get high recovery of the main production of CaO. Using Kissinger, Flynn-Wall-Ozaw, and Coats-Redfern methods, the mechanism model of phosphogypsum decomposition was confirmed, the decomposition process belonged to the core-form and growing mechanism (n = 2), and the kinetic model was d alpha/dt = 1.6 x 10(16)e-475.99 x 10(3)/RT x 2(1-alpha)[-ln(1-alpha)](1/2), where alpha is conversion.