Wet flue gas desulfurization was simulated to improve gypsum production using low-grade limestone. High-grade limestone with 94 wt% CaCO3 content is used for producing gypsum with 93 wt% purity, but owing to the resource depletion of high-grade limestone, low-grade limestone should be replaced as an alternative. However, lowgrade limestone with CaCO3 purity of less than 94% contains impurities such as MgCO3, Al2O3, and SiO2, which reduce gypsum purity. To resolve this issue, a process involving mixing of both low-grade and high-grade limestone was simulated to predict the quantity of low-grade limestone that could be utilized. Many reactions like limestone dissolution, SOX absorption, and crystallization were considered and were simulated by different models in Aspen plus. For process optimization, the following constraints were set: 93 wt% gypsum purity, 94% desulfurization efficiency, and 3,710 kg/h total limestone usage, which maximized the mass flow of low-grade limestone. The maximum blending quantity of low-grade limestone for 2,100 kg high-grade limestone that satisfied the constraints was ~1,610 kg.