The invention discloses a device and a method for combined removal of sulphur dioxide (SO2), nitrogen oxide (NOX) and mercury from fume. In the device, a nitrogen dioxide (NO2) generation device is added between a dust remover and a wet desulphurization tower in a wet desulphurization system. The method comprises the following steps of: adding 9 percent of NO2 gas into the fume which is subjected to dust removal by the dust remover by using the NO2 generation device; adjusting the content of NO2 in the fume to 50 percent of the total amount of the NOX; controlling the PH value of a circulating solution in an absorption tower to 5.5 to 6.3; absorbing and oxidizing the SO2 and the NOX in the fume by a reaction tower to obtain a mixed solution of ammonium sulphate and ammonium nitrate; carrying out operation on the mixed solution in an ammonium sulphate and ammonium nitrate recovery system to generate a fertilizer which can be used for agricultural production, wherein desulphurization efficiency is over 95 percent and denitration efficiency is over 80 percent; and after adding the NO2 into the fume, oxidizing a certain amount of mercury oxide (Hg) as Hg<2+>, absorbing the Hg<2+> by a water solution of the system, and removing from the fume, wherein the removal efficiency of the Hg can be increased by over 50 percent.