This paper proposes a catalyst to lower the decomposition temperature of phosphogypsum (PG) and investigates the reaction mechanism. PG is stable in the inert atmosphere, decomposed above 1100 degrees C, and unstable in the reducing atmosphere relatively. The PG decomposition temperature dropped to 800-1000 degrees C through previous study in a CO/coal reducing atmosphere, which is still high. The Ellingham diagram and FactSage6.1 were used as a guide to find catalyst to lower the PG decomposition temperature further and explore the reaction mechanism, respectively. The analysis of FactSage6.1 Reaction, Equilib, and Phase diagram module showed that Fe(+3) was reduced by CO and oxidized by S(+6) reacted in the molten state completely, which lowered the PG decomposition temperature significantly. PG powder was added to FeCl3 aqua and decomposed in a thermogravimetric analyzer (TGA). With different FeCl3 impregnation concentrations, CO flow rates, and heating rates, the decomposing properties of PG were investigated by thermogravimetric analysis (TG) and differential thermogravimetric (DTG). Impregnated by 0.8 mol/L FeCl3 and flowed 4 mL/L CO, the temperature of PG decomposition drops to 610 degrees C. This research proves that iron is a proper catalyst to lower the PG decomposition temperature and provides a new reaction path for PG decomposition through gas solid reaction, liquid solid reaction, and gas liquid solid reaction.