The invention relates to the field of chemical industry and in particular to a method for preparing nano calcium sulfate. The method comprises the following steps: adding a crystal form control agent into refined lime milk to produce a CaSO4 crystal nucleus, and fully emulsifying the lime milk; atomizing the lime milk to be sprayed into an absorption tower, introducing sintering flue gas and cool-humidified oxidation air into the absorption tower, thereby obtaining gypsum slurry, wherein the sintering flue gas needs to be subjected to dust extraction and purification and cooling humidification before introduction; adding a modifier into the gypsum slurry for performing surface modification treatment, so that the surface of the produced CaSO4 powder is hydrophobic; and preparing nano calcium sulfate through the gypsum slurry subjected to surface modification treatment. The nano calcium sulfate is directly produced by utilizing SO2 in the sintering flue gas, so that the environmental pollution problem is solved, and a comprehensive resource utilization aim is achieved.