Abstract: Biomass is considered as a zero-carbon, renewable energy source. Co-firing biomass fuels in large-scale power plants not only enables the highly efficient large-scale utilization of biomass energy but also significantly reduces CO₂ emissions. In this study, a co-firing experiment was conducted using salix psammophila biomass pellets in a 660 MW ultra-low emission coal-fired power plant, utilizing the existing pulverization and combustion systems of the power plant. Two combustion modes were proposed: “single mill and single burning”, where biomass was pulverized separately, and “single mill and mixed burning”, where coal and biomass were co-pulverized. The study analyzed the effects of co-firing biomass on the operation of coal mills, combustion characteristics of the plant, boiler thermal efficiency, pollutant emissions, and the characteristics of fly ash. The results indicate that the biomass pellet co-firing can be achieved using the existing pulverization and combustion equipment. In the “single mill and single burning” mode, a biomass feed rate of 15 t/h to 20 t/h is recommended, while a biomass co-firing ratio of 3% to 5% is optimal for the “single milling and mixed burning” mode. The HP coal mill demonstrates good adaptability to biomass pellets, and meets the operational requirements. The operating current of mill is lower when the biomass is milled alone or with coal. During the testing, the burners operate stably, with no premature ignition or slagging observed. When co-firing, the furnace temperature decreases slightly by about 15 ℃ to 32 ℃, while the temperature of flue gas at air preheater inlet shows little change. The thermal efficiency of boiler remains in the range of 92.83% to 93.93%, nearly equivalent to the thermal efficiency under pure coal conditions. The burnout performance of boiler is satisfactory when co-firing, with minimal impact on thermal efficiency. Oxygen content is identified as a key factor influencing NO_x formation. By adjusting the air supply parameters, co-firing biomass can reduce the formation of NO_x. The fly ash produced under various conditions with “single mill and single burning” meets the strength activity index requirements, with values exceeding 70%, and fulfill the standards for building materials.