The invention discloses a surface modification method for preparing anhydrous calcium sulfate whisker difficult to dissolve in water. The method comprises A: modifier screening and solution preparation, B: preparation of a calcium sulfate whisker turbid liquid, and C: roasting of the anhydrous calcium sulfate whisker. The method solves the problem that calcium sulfate dihydrate whisker or calcium sulfate hemihydrate whisker are unstable in a hydration process, and also improves the compatibility of the calcium sulfate whisker and the polymer material. The method comprises simple processes of immersion, filtration, dehydration and roasting. The prepared whisker is modified by a salt solution with a mass concentration of 0.5-10g/L through immersion so that the modified whisker has good dimensional stability, slightly solubility and good compatibility and a high retention rate in papermaking pulp. The method has the advantages of simple process, low production cost, non-toxicity of the modifying agent and eco-friendly preparation process. The sulfuric acid whisker can be used in the field of papermaking fillers and can also be used as filler in a resin material.