Abstract: Against the backdrop of the in-depth advancement of China’s dual carbon strategy, the transformation of the country’s energy structure has entered a critical phase, and coal-fired units, as the core pillar of power supply and one of the major sources of carbon emissions, have their carbon emission reduction progress directly bearing on the effective achievement of the dual carbon goals; at present, the technical pathways for carbon emission reduction of coal-fired units mainly focus on energy-saving and carbon reduction retrofits, clean energy substitution, carbon capture, utilization and storage (CCUS), among others, and coal-biomass co-firing has emerged as an important and feasible pathway for large-scale carbon emission reduction in the short term by virtue of its advantages such as high technical maturity, low retrofitting difficulty and controllable costs, thus attracting extensive attention in the energy field. Previously, scholars at home and abroad have conducted numerous fundamental studies on the feasibility, combustion kinetic characteristics and pollutant emission laws of coal-biomass co-firing, most of which have focused on small-scale laboratory tests or pilot platforms, but field test studies on large-scale ultra-supercritical units remain scarce, especially the lack of targeted test data adapted to specific coal types and regional biomass resources, which makes it difficult to provide accurate technical support for the co-firing retrofitting and stable operation of industrial coal-fired units directly. To fill this research gap and verify the industrial feasibility and carbon emission reduction potential of biomass co-firing in coal-fired units, a 660 MW ultra-supercritical lignite-fired unit in Inner Mongolia was selected as the research object, and briquetted sheep manure was chosen as the biomass fuel for field tests in light of local biomass resource endowments, focusing on investigating the performance changes of the pulverizing system under a 10.85% blending ratio of briquetted sheep manure, as well as the variation laws of unit combustion characteristics, carbon content in ash and slag, and pollutant (NOx, SO2) emissions under the mode of multi-mill coordinated co-firing at different load conditions (300~400 MW, 400~500 MW), while the carbon emission reduction benefits were quantitatively evaluated. The results show that co-firing of briquetted sheep manure causes no significant disturbance to the operation of the core equipment of the unit: under low-load conditions, the average current of the coal mill only increases by 1.2 A, and the current is basically consistent with that of the pure coal combustion condition under high-load conditions, the increase in the pressure difference between the inlet and outlet of the coal mill is less than 0.7 kPa which falls within the safe operation threshold, the furnace temperature and flue gas exhaust temperature are slightly lower than those of the pure coal combustion condition, and there is no obvious fluctuation in the operating parameters of induced and forced draft fans, attempering water consumption and furnace negative pressure; co-firing can slightly reduce the carbon content in fly ash and bottom slag to improve the combustion efficiency, and the pollutant emission reduction effect is optimized with the increase of unit load—under the 400~500 MW load condition, compared with the pure coal combustion condition, the concentrations of NOx and SO2 in flue gas decrease by 18.35 mg/Nm3 (a reduction rate of 9.68%) and 277.16 mg/Nm3 (a reduction rate of 7.78%) respectively, which is significantly better than those under the 300~400 MW load condition (with reduction rates of 6.23% and 4.49% respectively); quantitative calculation results show that when two coal mills maintain a 10.85% blending ratio of briquetted sheep manure, the annual CO2 emission reduction of the unit can reach 93,373.98 tons, demonstrating remarkable carbon emission reduction benefits.