Phosphogypsum (PG) is a solid waste (a byproduct of the "wet process" phosphoric acid production) in the phosphate fertilizer industry which is environmentally harmful. The thermal decomposition of PG to recycle the sulfur source and produce cement is a proper way for the utilization of PG. In the previous work of our group, the initial decomposition temperature of PG lowers largely with Fe catalyst and addition. In this paper, based on XRD (X-ray powder diffraction), XPS (X-ray Photoelectron Spectroscopy), and calculation of Fact Sage 6.1, the transformation of Ca, S in PG decomposition with Fe catalyst in CO atmosphere is explored further. There are a series of competitive gas-solid/solid-solid reactions. At a lower temperature (<1000 degrees C), the main existences of Ca are CaS and CaSO4 (anhydrite) during the PG decomposition. 1050-1100 degrees C is the interval area which the reaction of CaO, CaS competes intensively. The main product of PG decomposition is CaO and S exists as SO2 when the temperature is greater than 1100 degrees C. With a series of Fe oxides, the PG decomposition temperature lowers largely and the catalytic reaction system is built. The shrinking core model could properly imitate the process of PG decomposition with Fe catalyst in CO atmosphere.