The invention discloses a method and a device for removing mercury by waste water subjected to wet flue gas desulphurization in a coal-fired power plant. The method is characterized in that waste water subjected to wet flue gas desulphurization is treated by conventional waste water treatment processes, then is fed back to an upper part of a coal dust conveyor belt and is sprayed in coal dust, or is fed back to a part between an air preheater and a deduster and is sprayed in flue gas so that Hg in the coal dust or in the flue gas can fully contact with the waste water subjected to wet flue gas desulphurization and mercury removal effects are obtained. Compared with the existing mercury removal technology, the method provided by the invention does not need operation consumables, does not consume a heat source, has a low cost, overcomes the defect that the existing mercury removal technology needs a high operation cost and especially, the active carbon injection mercury removal technology needs a high operation cost, does not produce adverse effects on a boiler, dedusting operation, denitration operation and desulfuration operation, realizes humidification of fly ash in flue gas, improves deduster dedusting efficiency and a waste water recovery rate, saves a water resource, realizes treatment of waste with waste, and reduces a high cost of a chelating agent adopted in conventional waste water treatment.