The invention provides a process for preparing .alpha.-calcium sulfate hemihydrate which comprises the following steps: adding aqueous solutions containing sulfate ions and calcium ions to calcium chloride solution with a range of concentration; reacting the mixture of a) at a range of temperature for an appropriate time; filtering and washing the reaction products of b) with a rising solvent to isolate the solid and liquid phases; and d) collecting and drying the solid phase to obtain .alpha.-calcium sulfate hemihydrate with high purity; wherein the relationship between the range of the concentration of calcium chloride (y) and the range of the reaction temperature (x) is shown as the following formula: -0.4x.sup.5+7.33x.sup.4-52.83x.sup.3+187.17x.sup.2-328.27x+316