The invention discloses a method for catalyzing and oxidizing elemental mercury in flue gas by magnetic balls in fly ash, in order to realize the removal of elemental mercury in flue gas. The method comprises the steps of: firstly, performing magnetic separation to the fly ash separated and collected by a dust remover by using a magnetic separator, so as to obtain magnetic ball particles; then spraying the magnetic ball particles into a flue arranged in front of the dust remover, in order to mix the magnetic ball particles with the flue gas to be removed and carry out the catalytic reaction with the elemental mercury in flue gas so as to oxidize the elemental mercury into mercury oxide; and finally, introducing the mercury oxide into a downstream pollutant control apparatus along with the flue gas for post-process, thereby removing the elemental mercury in flue gas. The invention further discloses an apparatus for catalyzing and oxidizing elemental mercury in flue gas by magnetic balls in fly ash. With the adoption of the method and the apparatus provided by the invention, the elemental mercury in flue gas is catalyzed and oxidized by the magnetic balls in fly ash without using additional synthetic catalyst, so the oxidation and removal rate of mercury is high and the cost is low; and the use of the fly ash cannot be affected at the same time when the elemental mercury is subjected to catalytic oxidation.