Abstract: Co-firing biomass is one of the important pathways for achieving low-carbon retrofit of coal-fired power plant. To explore the feasibility of directly co-firing biomass pellets with coal in large coal-fired power plant, a field test using existing pulverizing systems for salix pellets pulverization was conducted in a 1000 MW coal-fired power plant for the first time. Milling characteristics and safety of the medium-speed mill in grinding biomass power plant for the first time. Milling characteristics and safety of the medium-speed mill in grinding biomass pellets, influence of biomass blending on the fly ash and fuel burnout characteristics were analyzed. Results show that biomass powder particle size of the sand willow pellets, after being ground by a medium-speed mill, is primarily concentrated below 1000 μm. The medium-speed grinding of sand willow pellets is feasible, and the maximum output of the grinding process can exceed 40% of the designed coal mill output. The results of combustion characteristics with biomass addition show that more large black and irregularly shaped particles in the fly ash was observed. And the increase of biomass co-firing rate decreased the proportion of particles within size ranging 100-180 μm, while increased the proportion of particles within size ranging 50-75 μm. Besides, the unburned carbon content in fly ash gradually increased with the increasing biomass co-firing rate. With biomass co-firing amount increasing from 25 t/h to 40 t/h, the fly ash carbon content increased from 0.77% to 1.01%. With biomass co-firing amount reached 45 t/h, the fly ash carbon content increased to 1.69%. Further analysis on the carbon content and burnout rate of biomass fly ash on basis of ash balance theory, demonstrated a trend of slight fluctuation followed by a significant decrease with the co-firing rate of biomass increasing. Within biomass co-firing rate ranging 25-40 t/h, the burnout rate remained above 99%. With biomass co-firing rate further increasing to 45 t/h, the burnout rate decreased slightly. Although the carbon content of biomass fly ash increases after co-firing, the actual burnout efficiency remains high due to the low ash content of biomass. Therefore, biomass co-firing has minimal impact on fuel burnout. The direct co-firing of salix pellets in large coal-fired power plant with medium-speed mills is highly feasible. However, the particle size of biomass powder and the co-firing ratio should be kindly regulated in order to ensure combustion efficiency and boiler operational stability. This study provides important field guidance for the direct co-firing of biomass in large coal-fired power plant in China.