Calcium sulfate whisker (CSW) was prepared from flue gas desulfurization (FGD) gypsum by recrystallization method, and then was employed in preparing ZnO/CSW photocatalysts by impregnation method. CSW and ZnO/CSW were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Xray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), UVVis diffuse reflectance spectroscopy (UV-Vis DRS) and photoluminescence spectrum (PL). The photocatalytic activity of various ZnO/CSW photocatalysts was evaluated by photocatalytic degradation of methylene blue (MB) under simulated sunlight irradiation. The results showed that various ZnO/CSW photocatalysts exhibited much higher photocatalytic activity than pure ZnO and CSW. Among various ZnO/CSW photocatalysts, the photocatlytic activity of ZnO/ CSW increased as increasing the ZnO loading amount from 1.2% to 8.7%, but decreased when the ZnO loading amount was beyond 8.7% due to the increasing crystalline size of ZnO and recombination of photogenerated hole/electron pairs. Besides ZnO loading amount, MB initial concentration and the dosage of photocatalyst also had significant influence on MB degradation rate, and MB degradation rate over ZnO/CSW reached 95.4% under optimum conditions. Kinetics study revealed that the photocatalytic degradation of MB over ZnO/CSW can be described by the pseudo-first-order kinetic model, and the apparent rate constant k versus ZnO loading amount L, MB initial concentration C0 and the dosage of photocatalyst D can be described as: k=0.5237L0.5193C0 -0.3074D0.4589.