The α-calcium sulfate hemihydrate whiskers were prepared by hydrothermal method with titanium gypsum as raw material, calcium hydroxide saturated solution as solvent and magnesium sulfate heptahydrate as crystal promoter. The effects of reaction temperature, reaction time, solid-liquid ratio of slurry, content of crystal promoter and volume of calcium hydroxide solution on the yield and morphology of α-calcium sulfate hemihydrate whiskers were investigated, the growth mechanism of whiskers was clarified, too. The results show that in alkaline hydrothermal environment, titanium gypsum first transforms into α-calcium sulfate hemihydrate and then grows into whiskers. Adhering to the existing whiskers is an important way of whisker growth. When the solid-liquid ratio of slurry is 1∶12, the temperature is 120 ℃, the reaction time is 8 h, and the content of crystal promoter is 6%, the α-calcium sulfate hemihydrate whisker has high yield and excellent performance. The whiskers yield with smooth surface can reach 71.6%, and the mean aspect ratio is 70.