The invention discloses a preparation method of an iron-manganese catalyst. The method comprises the following steps: adding manganese oxide nanoparticles into deionized water, ultrasonic dispersion for 40-50 min, adding a ferric nitrate solution, dropwise adding ammonia water under a magnetic stirring condition until the pH value is 2.4-2.6, transferring the mixed solution into a hydrothermal reaction kettle, putting the hydrothermal reaction kettle into a drying oven for hydrothermal reaction at 165-175 DEG C for 4-5 hours, separating and washing, drying the obtained solid at 65-75 DEG C for23-25 hours, finally calcining the solid at 380-390 DEG C in an oxygen atmosphere for 4-5 hours, and cooling. The method is simple, convenient, rapid and easy to operate, the prepared iron-manganesecatalyst has high NOx catalytic activity, and meanwhile, the catalyst shows excellent water resistance and sulfur resistance and can be prepared on a large scale.