Abstract: Natural gypsum was used as raw material in airtight reactor to prepare hemihydrate calcium sulfate whiskers by hydrothermal method, and SEM was utilized to observe the morphology and aspect ratio. The additive, particle size, hydrothermal temperature, pH, initial slurry concentration and hydrothermal time on the morphology of calcium sulfate whisker were investigated. The results showed that additive increased the solubility of natural gypsum in solution; particle size of raw material affected the dissolution rate of natural gypsum, smaller particles easily reached the supersaturated sate; it's difficult to turn into calcium sulfate whisker when hydrothermal temperature was less than 100 ℃; pH of solution can change the mass fraction of ion, and as pH increased the average diameter of whisker gradually decreased, along with being invalid of the additive of CuCl₂; the higher slurry concentration was, the smaller the space of whisker growth was and the slower ion transport rate was. The optimum conditions for the preparation were as follows: the diameter of raw material was 4 μm with the slurry concentration at 5%, the content of copper chloride was 1.5%, temperature was 120 ℃, pH=5.3, and reaction time was 140 minutes. Then, the calcium sulfate whisker of average diameter 1~2 μm, aspect ratio 160 were obtained.