The invention provides a method for preparing a gypsum whisker. The method comprises the following steps of: (1) drying limestone sludge to obtain limestone powder; (2) adding a certain amount of limestone powder to dilute sulphuric acid for reaction to generate calcium sulphate; (3) filtering the calcium sulphate obtained from the step (2), then adding water to prepare a calcium sulphate solution and controlling the concentration of the calcium sulphate solution, then adding zinc sulphate as a catalyst for hydrothermal reaction, and filtering to obtain a hemihydrate gypsum whisker; (4) drying the hemihydrate gypsum whisker to obtain an anhydrous gypsum whisker. The method for preparing the gypsum whisker, which is provided by the invention, changes the wastes into valuables by utilizing the limestone sludge regarded as waste residues and waste sulphuric acid liquor which are generated in the steel industry to prepare the anhydrous gypsum whisker with excellent properties. The method disclosed by the invention can not only be used for reducing the production cost of the gypsum whisker, but also be used for protecting the environment, thereby generating preferable influence on the harmonious development of society and environment.