A catalyst with high efficiency and low-cost is proposed which can catalyze hydrogen peroxide (H2O2) for desulfurization and denitrification simultaneously in this study. The active catalyst made by fly ash of power plant was prepared through milled, alkali modification and magnetic separation. Electron-spin-resonance test confirmed that it catalyzed the decomposition of H2O2 to generate hydroxyl radicals (center dot OH). The simultaneous removal efficiency reached 100% for SO2 and 80% for NOx under the optimal conditions (molar ratio of H2O2 to NOx = 3:1; reaction temperature = 130 degrees C). The catalyst was characterized by X-ray fluorescence, X-ray diffraction, Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Photoluminescence and so on. The NOx removal efficiency was affected by center dot OH concentration which attributed to content of iron in fly ash. Magnetic separation increased the iron content, and alkali addition benefited this process. We proposed reasonable catalytic mechanisms with catalyst/H2O2 in low- and high-temperature zones. In the low-temperature zone, the dissolved iron ions catalyzed H2O2 decompose to generate center dot OH. In the high-temperature zone, H2O2 was catalyzed by hydroxyl iron (FeOH) on the catalyst surface.