Because of higher thermal load and flame center temperature of the flame center,the generation of thermal NOx is increased. The down-fired boiler has higher NOx mass concentration than other coal boilers，and the selective catalytic reduction (SCR) denitrification device at the rear of the boiler has high requirements. To investigate the influences of flow field and species concentration profile on deNOx efficiency and ammonia slip in an industrial SCR reactor,a 3D computational fluid dynamics (CFD) model of SCR device was established based on ANSYS FLUENT software,so as to improve the denitrification performance of device,and numerical simulation was carried out.In order to make the calculation results closer to the actual conditions,the actual NOx concentration of the boiler under full load was used as the SCR inlet boundary conditions in the numerical simulation. Through detailed analysis of flow characteristics of SCR system such as flue gas velocity distribution,component concentration distribution,ammonia escape,etc.,simulation and optimization were carried out for the influence of flow field optimization,zoned ammonia injection and new static mixer on denitration performance,so as to improve the flow field and concentration field uniformity of SCR system. The simulation results show that by increasing the number of deflectors in the top chimney flue,the uniformity of catalyst inlet velocity distribution is improved,and the strip area with high velocity can be eliminated. After optimization,the relative standard deviation of catalyst inlet velocity distribution is reduced from 20.86% to 3.80%. The influence of uneven NOx concentration distribution at the inlet can be eliminated by adding a static mixer with triangular airfoil,and SCR uniformity of the concentration field can be improved. The relative standard deviation of the maximum ammonia nitrogen molar ratio is reduced from10.72% to 1.78%.The reconstruction scheme is obviously superior to the original design,which has important guiding significance for engineering design.