The invention relates to a method for preparing calcium sulphate crystal whisker by using phosphogypsum, which belongs to the technical field of phosphorous chemical preparation. Solid drogs phosphogypsum is used as raw material; after a synthetic reaction, filtering, heating and dissolving, thermal filtering and cooling crystallization for the raw material, the calcium sulphate crystal whisker product with the controllable diameter and length is prepared, the diameter of the crystal whisker is 1-4mum and the length of the crystal whisker is 50-200mum. The adoption of the solid drogs phosphogypsum for preparing the calcium sulphate crystal whisker reduces the production volume of natural plaster, saves resources, turns the waste into benefit, lowers the production cost of the calcium sulphate crystal whisker and improves added value of the product in the process of utilizing the phosphogypsum. The obtained calcium sulphate crystal whisker has excellent property, low cost and wide application.