Abstract: A 600 MW coal-fired unit with one more layer of backup catalyst is taken as an example in this paper to measure the NO_x concentration distribution at the SCR reactor outlet, the NO_x concentration deviation between the outlets of the SCR reactor and the FGD tower, and the concentration of ammonia slip, for the purpose of improving the operation quality of the thermal power plant after the ultra-low emission technical transformation. Some problems, such as the ammonia injection optimization and blocked ammonia branch nozzle dredging, are resolved and the corresponding adjustment is applied. The low NO_x burner reformation decreases the NO_x concentration at the SCR inlet. To avoid adverse effects due to excessive ammonia injection, it is suggested that the denitrification efficiency should be lowered accordingly, i.e. to control the ammonia injection to a certain level based on the NO_x emission standard. The catalyst activity of the unit is evaluated, and the constraint indexes of catalyst replacement, installation or regeneration strategy selection are proposed.