The anhydrous calcium sulfate (AH CaSo(4)) whiskers were prepared in H2O-H2SO4 autoclave-free hydrothermal system by using flue gas desulfurization gypsum as raw materials. The influences of solid/solution ratio, H2SO4 concentration, and surface modification agent (stearic acid) on AH whiskers growth were detected. The whiskers, with a smooth surface and high aspect ratio, were obtained in a 20 mass% H2SO4 solution for hydrothermal 1 h with 1:60 ratio of solid/solution. The whiskers diameter hardly changed, but the amount increased in the sample by adding stearic acid. The whiskers were not obtained in 10 mass% H2SO4 solution for hydrothermal 1 h with 1:60 ratio of solid/solution. Only a little of whiskers were obtained in 30mass% H2SO4 solution at the same hydrothermal condition.