Phosphogypsum (PG) is a waste byproduct from the processing of phosphate rock by the "wet acid method" of fertilizer production. One of the main methods for reusing PG is to decompose and recycle Ca and sulfur contained in it. However, the decomposition reaction process is very complex because of its complicated contents, and very high temperature is needed for the reaction. In this paper, to decrease the reaction temperature, CO as a main reducer and some additives were added in the decomposition process. Results show that the decomposition temperature will decrease from 1000 to 809 degrees C with pure CO. When CaCl2 is used as an additive, the decomposition temperature can decrease to 790 degrees C, and at the same time the reaction rate will be increased, the main product being CaS at this condition. The thermal and kinetic action of this process has also been discussed.