Abstract: In order to study the effects of sludge reburning ratio, moisture content, and boiler load on the combustion characteristics of coal-fired boilers, a 600 MW coal-fired boiler was selected as the research object. The boiler burner was modified by adding a reburning zone between the main combustion zone and the burnout zone to investigate the effects of sludge reburning on the boiler temperature field, composition field, and pollutant NO_x emission concentration. The results showed that sludge reburning caused a decrease in the temperature of the main combustion zone of the boiler, but an increase in the flue gas temperature at the furnace outlet, resulting in a decrease in the concentration of pollutant NO_x. As the proportion of sludge reburning increases, the volume fraction of O₂ at the furnace outlet gradually decreases, and the volume fraction of H₂O gradually increases. When the reburning proportion is not higher than 10%, the furnace combustion is stable, and the temperature change trend in the main combustion zone is close; When the reburning ratio reaches 15%, sludge reburning has a significant impact on the combustion stability of the furnace, and the CO content in the upper part of the furnace increases significantly; The moisture content of sludge increased from 10% to 35%, and the overall temperature of the furnace decreased by 10–15 K. The volume fraction of O₂ at the furnace outlet did not change much, while the volume fraction of H₂O increased from 8.82% to 11.8%; The NO_x at the outlet of sludge with a moisture content of 10% must be lower than that of sludge with a moisture content of 35% in the combustion condition; When the boiler load is reduced to 70% and 50%, adding sludge for re combustion reduces the NO_x mass concentration at the furnace outlet from 424.57 mg/m³ to 341.75 mg/m³ and 418.74 mg/m³ to 354.76 mg/m³.