The invention provides a device and a method for producing magnesium sulfate by using flue gas of a coal-fired boiler, and particularly provides a device and a method for producing the magnesium sulfate by direct crystallization in a magnesium oxide-process waste desulfurization liquid tower by using the flue gas, low in oxygen content, of the coal-fired boiler. The device comprises oxygenation equipment, flue gas desulfurization equipment, slurry circulating equipment, evaporation and concentration equipment, cyclic sedimentation equipment, oxidization equipment, crystallization equipment, centrifugation equipment, drying equipment and the like. By adopting the device and the method disclosed by the invention, the desulfurization efficiency and the quality of a side product magnesium sulfate can be guaranteed under a condition that the wind power consumption and steam consumption of an oxidization fan are greatly reduced, the operating cost of a magnesium sulfate production system is greatly lowered, and the desulfurization operation cost is lowered.