In an eco-friendly technology, MgSO4 center dot 7H(2)O is produced as a by-product in the magnesium-based wet flue gas desulfurization (FGD) process. Equilibrium data for the quaternary system of MgSO4-MgCl2-CaSO4-CaCl2-H2O at 308.15K, 338.15K, and 368.15K were investigated using the method of isothermal solution saturation. The dry-salt phase diagram of the system mainly includes the three-salt co-saturated point, solubility isotherm curves, and crystallization regions. The results could provide assistance for producing MgSO4 as a by-product with evaporation crystallization in the magnesium-based wet FGD process.