Calcium sulfate hemihydrate crystals with different morphologies were prepared in sodium hydroxide-distilled wa-ter system by hydrothermal method using calcium sulfate dihydrate as raw materials.The mechanism for influence of OH^- on the crystalmorphology of hydrothermal products was investigated by XRD,XPS and conductivity etc..The results showed that calcium sulfate hemihydrate crystals with different morphologies can be prepared under alkaline condition.The content of fibrous hydrothermal products decreased and the content of short columnar hydrothermal products increased with the increase of OH^- concentration.And when the concentration of OH^- was 1.0×10^-2mol/L,the morphology of hydrothermal products was mainly short column.The reason for the influence of OH^- on the morphology of calcium sulfate hemihydrate crystals was as follow：the consumption of Ca²⁺ by OH^- in solution promoted the dissolution of calcium sulfate dihydrate and the decrease of Ca²⁺ concentration was not conducive to the nucleation and growth of calcium sulfate hemihydrate crystals.Meanwhile,the promoting effect of OH^- on the dissolution of calcium sulfate dihydrate can increase the concentration of SO₄^2- in solution,thus promoting the growth of calcium sulfate hemihydrate crystals along（110） and（200） crystal planes.In addition,OH^- selectively adsorbed on（002） crystal plane and reacted with Ca²⁺ on the crystal plane to form Ca（OH）⁺ and Ca（OH）₂,which hindered the growth of calcium sulfate hemihydrate crystals along the crystal plane.Under the above three effects,the morphology of calcium sulfate hemihydrate crystals can transition from fibrous to short columnar shape.The study can provide theoretical basis for producing calcium sulfate hemihydrate crystals with different morphology under alkaline condition.