The invention relates to a method for preparing high-purity dihydrate gypsum whiskers by using an industrial byproduct calcium chloride. The method comprises the following steps: 1) preparing a calcium chloride solution from an ammonia-soda-process industrial byproduct calcium chloride slurry, heating the calcium chloride solution to 30-80 DEG C, and adding a nucleation accelerator and a crystal form control agent to obtain a reaction mother solution; and 2) adding a 40-60-DEG C sodium sulfate solution into the reaction mother solution, stirring to react for 30-40 minutes, keeping the temperature for 0.5-3 hours to obtain a dihydrate gypsum whisker slurry, and carrying out after-treatment to obtain the high-purity dihydrate gypsum whiskers. The surplus byproduct in the salt chemical industry production process is used as the raw material to prepare the dihydrate gypsum whiskers, of which the length-diameter ratio is 10-100, the crystal length is 50-500 mu m, the purity is 98% or above and the whiteness is 90 or above. The invention provides a high-added-value utilization scheme for the calcium chloride slurry byproduct; and the obtained gypsum whisker can be widely used in the fields of papermaking, chemical industry, novel construction materials, composite materials and the like.