Aiming at utilization of poorly soluble potassium slate, recovery and water leaching kinetic of potassium salt in the roasted product of potassium slate-FGD gypsum-calcium carbonate system were studied. The results showed that the main components in the roasted product were Ca2SiO4, CaO, K2SO4 and Ca3Al2O6. K2SO4 in the roasted product could be separated and recovered by water leaching under stirring and fractional crystallization. When the ratio of liquid to solid was 2 mL/g, K+ concentration of the leaching solution 14.98 g/L and K2SO4 content of the crystallized substance was 91.28%. On-line monitoring of specific electrical conductivity of the leaching system verified the prediction that leaching kinetics of K2SO4 from the roasted product fit to the dissolution model. Higher temperature, higher stirring speed and smaller particle size of roasted product could increase the leaching rate. On this basis, by multiple linear regression, the leaching rate was expressed as dc/dt=(9.567′10-3T-1.699′10-3d+7.561′10-4r-2.668)(0.0722-c).