Calcium sulfate whiskers (CSWs) were synthesized using purified flue-gas desulfurization (FGD) gypsum as raw materials using a hydrothermal method, where the filtrate was recycled back to the reaction slurry. The study investigated the impact of pH and Cu2+ concentration in the reaction solution with or without reagent compensation. The effects of cycle number on the crystal morphology, phase structure, and productivity of the hydrothermal products were also examined. The findings indicate that the crystal morphology and quality of the CSWs deteriorated as the cycle number increased due to a rise in pH and a decrease in Cu2+ concentrations in the reaction solution without reagent compensation. However, adding a specific amount of H2SO4 and CuCl2 to the slurry with filtrate recycle maintained the long fibrous shape of the hydrothermal products for up to seven cycles while also increasing their productivity from 73.7% to 86%. Regardless of reagent compensation, the cycle number of filtrate significantly affected the crystal morphology of the hydrothermal products, but it had no impact on their phases.