The solubility of alpha-calcium sulfate hemihydrate in CaCl2-MgCl2-KCl solution at temperatures from (353.0 to 371.0) K was measured by the dissolution equilibrium method. The investigated concentration ranges of CaCl2, MgCl2, and KCl were up to (6.01, 4.50, and 0.27) m, respectively. The calcium ion demonstrates the common ion effect, which results in the decreased solubility of alpha-calcium sulfate hemihydrate in CaCl2 solution. The solubility of alpha-calcium sulfate hemihydrate is augmented due to the association effect caused by the formation of stable MgSO40 in MgCl2 solution. Also it is shown that potassium chloride can increase the solubility of alpha-calcium sulfate hemihydrate slightly, which may be attributed to the salt effect. In the mixed salt solution, the solubility of alpha-calcium sulfate hemihydrate decreases because of the dominating action of the common ion effect. Compared with the solubility of calcium sulfate dihydrate in the mixed solution, the solubility of alpha-calcium sulfate hemihydrate is lower than that of calcium sulfate dihydrate above 363.0 K, which is beneficial to the conversion of flue-gas-desulfurized gypsum into alpha-calcium sulfate hemihydrate.