The invention relates to the technical field of sulfur and mercury removal, in particular to a method for removing sulfur and mercury in flue gas by using ethylenediamine; pyrolusite is processed into powder, water is added to the powder to obtain slurry, ethylenediamine is added to the slurry, ultrasonic treatment is performed, the content of manganese dioxide active ingredients in pyrolusite is increased, the treated slurry quickly reacts upon contact with sulfur dioxide and elemental mercury in flue gas, the sulfur dioxide is oxidized into sulfuric acid, and the elemental mercury is oxidized into ionic mercury; therefore, sulfur and mercury in flue gas are removed, and the removal rate of sulfur and mercury in flue gas is increased to 90% and above.