To meet the increasingly stringent air pollutant emission standards for the cement industry, our company adopted a series denitrification technology route consisting of "duct furnace staged combustion denitrification + iSNCR + SCR" to upgrade the denitrification system of a 5000t/d cement clinker production line. This retrofit achieved full-process denitrification control covering the source, process, and end-of-pipe stages. Before the retrofit, the production line was only equipped with an intelligent selective non-catalytic reduction (iSNCR) denitrification system. Although NO_X emissions below 100mg/m³ could be achieved, further tightening of emission limits would lead to a substantial increase in ammonia consumption and ammonia slip. After the retrofit, the NO_X concentration at the stack outlet was stably kept below 50mg/m³, the measured ammonia slip was below 8mg/m³, and the ammonia consumption per ton of clinker decreased by about 1.8kg/t.cl. Meanwhile, the firing system operated stably, and the clinker output and quality were not affected. This combined process effectively reduces denitrification costs, achieves low-cost ultra-low emissions, and provides a feasible technical pathway for deep NO_X abatement in the cement industry.