To effectively solve the problem of recovery and utilization of flue gas desulfurized gypsum (FGDG), a new method for preparing SO2 gas by adding a mixture of FGDG and pyrite into a boiling furnace and utilizing its high-temperature waste heat is proposed. Using the FactSage 6.1 Reaction module and the Equilib module, the pyrolysis behavior of FGDG with pyrite and the release characteristics of SO2 gas were studied. The effects of FeS2 addition, temperature, and ventilation flow rate on the decomposition of FGDG and the change in gas behavior were studied by thermogravimetric mass spectrometry and settling furnace (SF) experiments. In addition, the activation energies of the chemical reactions were determined by the Kissinger method, the Kissinger-Akahira-Sunose method, and the Flynn-Wall-Ozawa method. The results show that between 100 and 200 degrees C, mainly the loss process of CaSO4 center dot 2H(2)O crystal water from FGDG occurred. At 570 degrees C, FeS2 primarily undergoes self-decomposition. Between 820 and 1000 degrees C, the decomposition product of FeS2 reductively decomposes CaSO4. The average activation energy for the chemical reaction is 298.44 +/- 8.83 kJ/mol. A lower pressure and ventilation and a higher FeS2 addition and temperature are beneficial for the reduction and decomposition of FGDG and for the increase in SO2 concentration. Therefore, in this study, the feasibility of adding FeS2 to reduce and decompose FGDG to prepare SO2 was preliminarily verified. The technology proposed in this article may shed some light on the recovery and high-value utilization of FGDG.