In order to explain the growth mechanism of desulphurized gypsum whisker in hydrothermal environment and improve its high value-added utilization, calcium sulfate whisker (HH-CSWs) was synthesized in sulfuric acid solution at 120 ℃ for 60 min under hydrothermal conditions. XRD curves of HH-CSW and desulfurized gypsum were fitted and refined by using Rietveld method. According to the Rietveld fitting results, the crystal structure model of HH-CSW and desulfurized gypsum was established. Their crystal structures are obviously different in two aspects. On one hand, there are 6 Ca-O-S bonds and 2 Ca-O-H bonds in the desulfurized gypsum structure, while there are 8 Ca-O-S bonds in the HH-CSW structure. On the other hand, the desulfurized gypsum structure has two H₂O channels (along the c and a axes, respectively), whereas the HH-CSW structure has only one H₂O channel. It can be concluded that, the formation of HH-CSW prepared by using hydrothermal method in flue gas desulfurization (FGD) is through the two Ca-O-H bonds in the structure of desulphurized gypsum break to release H₂O and the combination with S to form two new Ca-O-S bonds, so as to release H₂O molecules along the c-axis and lose part of the crystalline water to form HH-CSW. This can be used as a theoretical basis for preparing desulfurized gypsum whisker with high ratio of length to diameter.