Abstract: Calcium oxide(CaO) was prepared by directly pyrolyzing the precursor called calcium carbonate (CaCO₃),which was synthesized by solid-state ball milling reaction between NH₄HCO₃ and CaSO₄·2H₂O,the byproduct generated during the preparation of MnSO₄.The effects of NH₄HCO₃-to-CaSO₄·2H₂O molar ratio,milling time,and ball-to-feed mass ratio on the conversion of CaSO₄·2H₂O were investigated respectively.The product was analyzed by X-ray diffraction and chemical analytical method.The reaction mechanism was also discussed.When the NH₄HCO₃-to-CaSO₄·2H₂O molar ratio is 3.5:1,the milling time is 40 min,and the ball-to-feed mass ratio is 5:1,the conversion rate of CaSO₄·2H₂O reaches 99.8%.Calcium oxide,whose purity is 99.2%,can be prepared by pyrolyzing the precursor at 1000℃for 1 h.The effectiveness of the solid-state reaction is improved by refining the reactant size, increasing the contact area,and providing necessary energy during the ball-milling process at ambient temperature.