A flue gas treatment device (1) is provided. A wet electrostatic precipitator (11) and a flue gas heater (13) are integrated in an integrated flue housing (14) of the flue gas treatment device (1), thus the occupied area of the flue gas treatment device (1) is smaller than that of the solution with devices being arranged separately. Furthermore, since a bidirectional transition flue is not required to be arranged in the integrated flue housing (14), the flue gas has a good flow uniformity, and further it is not required to arrange a flow equalization orifice plate in the flue, thus the flue gas has a small resistance, thereby reducing the power consumption of the draught fan and increasing the efficiency of the power plant.