The addition of calcium sulfate in the procedures of preparing flue gas desulfurization absorbent from calcium oxide and coal fly ash causes a negative effect in the slaking step and a positive effect in the following hydrothermal treatment step. The role of calcium sulfate added in each step was investigated. For the slaking step, XRD, XAFS, and Raman spectroscopic studies revealed that the addition of calcium sulfate in the step of slaking CaO with coal fly ash resulted in the coverage of calcium hydroxide with calcium sulfate to decrease the reactivity of the calcium hydroxide toward coal fly ash to form calcium silicate. For the hydrothermal treatment step, measurements of XRD, surface area, and Raman spectrum revealed that the addition of calcium sulfate promotes the formation of calcium silicate by suppressing crystal growth of calcium hydroxide to keep the reactivity of the calcium hydroxide toward coal fly ash.