Abstract: Biomass energy is a circular and renewable energy source, which can achieve sustainable development. In order to save coal resources, the use of biomass in coal-fired power plant boiler was explored to study the feasibility of mixed combustion of biomass energy and coal. Taking the 1 000 MW ultra-supercritical swirl boiler in a power plant as research object, we used Gambit software to build the model, and introduced Fluent software to study the pure coal burning condition and biomass mixed combustion condition with 20% biomass. The mixed combustion conditions include five working conditions of biomass being injected from different nozzle positions. By analyzing the change trend of furnace speed field, temperature field, flue gas component and pollutant NOx in each working condition, the best combustion condition was determined. The results show that compared with pure coal burning, in the process of mixed combustion, the highest temperature in the furnace is reduced, which improves the temperature of the furnace outlet, reduces the O₂ concentration of the furnace outlet, reduces the emission of NO, and promotes the combustion of fuel. When comparing the various mixed combustion conditions, the most significant reduction in pollutant NO emissions is achieved when biomass is injected from the uppermost burner without changing the boiler structure.