The invention provides a method directly using acid and alkaline activation of red mud to prepare an iron type denitrifying catalyst. The method comprises the following steps of (1) adding acid into the red mud, and reacting at the temperature of 70 to 200 DEG C, so as to obtain first slurry, namely acid slurry; (2) adding alkaline matter into the first slurry in step (1), and adjusting the pH (potential of hydrogen) value to 5 to 10, so as to obtain second slurry; (3) separating solid and liquid of the second slurry in step (2), so as to obtain filter cake; roasting the filter cake, so as toobtain an iron, titanium, silicon and aluminum oxide catalyst with denitrifying activity. The method has the advantages that the waste of red mud can be treated in a harmless way, and the waste is changed into the valuable; when the waste of the red mud is directly applied to the denitrifying process of flue gas, the existing denitrifying cost is greatly reduced, and the wide industrial application prospect is realized; the obtained iron, titanium, silicon and aluminum oxide catalyst can be applied into the denitrifying process of the flue gas of fired coal, and be used for replacing the existing vanadium, tungsten and titanium denitrifying catalyst.