An iron-based catalyst with excellent performance and low price was used for simultaneous removal of SO2 and NOx in a typical Fenton system. The catalyst was developed from fly ash of pulverized coal furnace by alkali-magnetic modification process. In other words, the raw fly ash was first modified by Ca(OH)(2), which destroyed the vitreous shell and made the internal substances such as Al2O3, SiO2, and Fe2O3 dissolve out gradually, and then separated magnetically and roasted, and the high magnetic Fe2O3 content of high reactivity was obtained. In the fixed bed experimental system, an ultrasonic nebulizer was adopted to atomize hydrogen peroxide which can reduce the particle size of hydrogen peroxide and shorten the gasification time greatly. Effects of the concentration of hydrogen peroxide, reaction temperature, iron oxide content, and initial concentration of SO2 and NOx on the collaborative removal efficiencies were investigated. Under low hydrogen peroxide concentration (1 mol/L) and flow rate (0.03 mL/min), 100% desulfurization efficiency and 90% denitrification efficiency can be achieved using hydrogen peroxide catalyzed by the modified fly ash. The catalytic mechanism and reaction mechanism were analyzed by XRF, XRD, ESA, SEM, BET, and XPS.