Abstract: Sulfur trioxide is one of the key pollutants in the flue gas of coal-fired units,which has an important influence on the stable operation,air quality as well as human health. In this paper,the research results about control of SO_2 oxidation over commercial SCR denitrification catalyst were summarized,including physical methods such as pore structure adjustment,isolation layer,wall thickness reduction and morphology adjustment,as well as chemical methods such as carrier,auxiliary agent,active component and surface acidity optimization.Due to the influence of kinetic factors( diffusion control),the denitrification reaction only occurs within the thickness of catalyst surface( denitrification area) of 75-100 μm,while the SO_2 oxidation reaction can occur within the whole wall thickness( area),so reducing the number of active components involved in the SO_2 oxidation reaction is the main technical content of the physical scheme. In vanadium denitration catalyst,the active component of V2 O5 can selectively catalyze the reduction of NOx,can also be the catalytic oxidation of SO_2 to SO3,and regulate the interaction of active component and carrier,and make the NH3 and SO_2 in the catalyst surface to form competitive adsorption,inhibit the catalyst for SO_2 adsorption,and reduce the redox capacity of the active species,which is an effective method of chemical solutions. The physical scheme is more suitable for plate denitrification catalyst,while the chemical scheme has no special requirements for catalyst structure,and the chemical scheme has better technical effect than the physical scheme. Based on the fact that denitrification belongs to a gas-solid heterogeneous catalytic reaction system,the catalyst formulation and physical structure were optimized and adjusted from the perspectives of the diffusion,adsorption,redox reaction of ammonia,SO_2 and other reactant,so as to achieve the balance between the denitrification activity K( NOx) and the oxidation activity K( SOx) of SO_2. In the development of commercial denitrification catalyst with low SO_2 oxidation rate,the influence of diffusion,adsorption,redox ability and other factors on the performance of the catalyst can be comprehensively considered. Meanwhile,the various schemes are adopted to modify the catalyst to significantly reduce the SO_2 oxidation rate of the catalyst under the condition of ensuring the denitrification performance of the catalyst. Under the requirement of ultra-low emission of coal-fired units,the amount of denitrification catalyst used or the content of active components in catalyst increased significantly,and the oxidation rate of SO_2 by denitrification system increased from 1% to 1.5%. At the same time,the oxidation rate of SO_2 of the new denitrification catalyst should be reduced by more than one third considering the pressure of SO3 removal by the back-end treatment equipment under the requirement of ultra-low emission.