The invention discloses a method for cooperatively promoting agglomeration growth of PM (particulate matter) 2.5 and treating desulfuration wastewater in an evaporation manner and a device of the method. The method particularly includes steps that agglomeration liquor prepared by desulfuration wastewater, high polymer adhesive and wetting agent is sprayed into an inlet flue of an electric precipitator and is evaporated by the aid of heat of flue gas, in an evaporation process, the wetting agent promotes dust to enter drops of the agglomeration liquor, the high polymer adhesive is connected with PM2.5 via a polymer chain with polar groups, the grain diameter of the PM2.5 is prompted to be enlarged, so that the PM2.5 can be gathered by the follow-up electric precipitator, the desulfuration wastewater is evaporated completely, suspended matters and dissolvable pollutants in the wastewater are converted into solid to be dissolved out, and the suspended matters and the dissolvable pollutants in the wastewater, together with dust in the flue gas, are gathered by the electric precipitator. The device consists of a boiler, an air preheater, the electric precipitator, a desulfurizing tower, a hydraulic cyclone, a desulfuration wastewater clarification tank and an agglomeration liquor preparation tank, and a double-fluid atomizing nozzle is arranged in the flue between the air preheater and the electric precipitator.