Due to the problems of unstable SCR performance and excessive ammonia slip in coal-fired power plants,taking the SCR denitration system of a representative 660 MW supercritical pulverized coal boiler as an example,and combined with the characteristics of ammonia injection grid and stationary vortex ammonia injection mixer,the multi-dimensional optimization and transformation of the stationary vortex denitration ammonia injection mixing device was studied,and the process of flue gas flow,change and mutual mixing at various positions in the flue was analyzed. An optimization based on multi-effect staged mixing,zoning vortex ammonia injection and forced rectification was proposed. According to the CFD calculation results,the optimized design scheme of the SCR reactor system proposed in this paper can fully achieve the requirements of uniform flow field distribution and NH₃/NO_x mixed distribution. At the cross section in the first catalyst layer inlet,the coefficient of variance for flue gas flow velocity is less than 15%,and coefficient of variance for NH₃/NO_x mixing is less than 5%. The incident angle of the flue gas reaching the first catalyst layer is less than 10°. After the modification of the SCR reactor device,the overall distribution of NO_x concentration at the outlet of the denitrification device is uniform,the average concentration is lower than the ultra-low emission limit,and the ammonia escape concentration value is much lower than the design value 3×10^-6. The practical test results further verify the reliability of optimization method. Therefore,the upgrade idea and theoretical research method are of great significance to guide the ultra-low emission transformation of SCR system of large coal-fired units.