Selective catalytic reduction by methanol (Methanol-SCR) on FER zeolite is a promising technology for the deNOx treatment of flue gas with a high concentration of sulfur. However, the resistance to alkali metal and sulfate ions is crucial for the practical application of Methanol-SCR catalysts. In this study, we investigated the poisoning mechanism of H-FER zeolite by K+ and sulfate ions. It was observed that K+ ions induce deactivation at high temperatures, whereas the decline at low temperatures (< 320 °C) can be attributed mainly to sulfate ions. Furthermore, surface modification of H-FER zeolite with metal oxide nanoparticles, such as alumina, has proved to be an effective method for improving its potassium sulfate resistance performance by preventing the diffusion of sulfate into zeolite pores.