The invention provides a process for preparing > calcium sulfate hemihydrate which comprises the following steps: (a) adding aqueous solutions containing sulfate ions and calcium ions to calcium chloride solution with a range of concentration (b) reacting the mixture of (a) at a range of temperature for an appropriate time (c) 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 > -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: -0.4x5+7.33x4-52.83x3+187.17x2-328.27x+316 < y < 1.2x5-22x4+158.5x3-561.5x2+984.8x-588 (2 < X < 5).
