The invention discloses a device and a method for preparing ammonium persulphate with an ammonia and urea solution flue gas purified absorption solution. The device essentially comprises a vessel body, a negative plate, a positive plate, a membrane, an anolyte storage tank, an anolyte supply pump, an anode flow meter, a catholyte storage tank, a catholyte supply pump, a cathode flow meter and a DC voltage-stabilized stable-state power supply. The positive plate and the negative plate are arranged opposite to each other, and are both connected to the power supply to generate an electric field between the negative plate and the positive plate. An electrolyzer is used as the basis for preparing the ammonium persulphate with a desulfurization and denitrification absorption solution. The anolyte and the catholyte are separated by the flat-plate type membrane. With such improvements as well as properly controlled reaction voltage, temperature and time, ammonium sulphate products with low added value can be effectively converted into ammonium persulphate with high added value. The method for preparing the ammonium persulphate with the ammonia and urea solution flue gas purified absorption solution is easy for operation, applicable to the processing of the desulfurization and denitrification absorption solution, eco-friendly and highly efficient.