The invention provides a device and a method for removing elemental mercury in coal-fired flue gas by using of desulfurization waste water. The device includes a denitrification device and a dust removal device, and also includes an air preheater and a limestone gypsum desulfurization device, and the denitration device, the air preheater, the dust removal device, and the limestone gypsum desulfurization device are sequentially connected through a pipeline, and the limestone gypsum desulfurization device is connected with the denitration device through a desulfurization pump, and an atomizationflue is also arranged between the denitrification device and the air preheater. One end of the atomization flue communicates with the desulfurization pump, and the other end of the atomization flue communicates with the air preheater; the method removes the elemental mercury in the flue gas by using of the device. The position of an injection point of the desulfurization wastewater is set beforethe air preheater, and the injection temperature range is 260 and 400 DEG C, the specific resistance of fly ash is reduced, the device is favorable for the capture of the fly ash, and by simple use ofthe desulfurization wastewater for removing the elemental mercury, secondary pollution is prevented, and the cost is reduced.