Abstract: To analyze the factors influencing the slagging behavior of a 660 MW swirl-opposed coal-fired boiler, numerical simulations were carried out to its combustion process respectively under 100%, 75% and 50% unit load, with coal particles of different sizes. Results indicate that in the case of 100% load, slagging mostly tends to be formed in the middle area of both the right and left wall at elevation 19-24 m, and in the case of 75% load, slagging is most probable to be formed in the middle area of both the right and left wall at elevation 21-23 m, and for the 50% load, slagging is least probable in the furnace. For coal particles of size 10 μm, slagging tends to be formed in the middle area of both the right and left wall at elevation 23-28 m, and the finer the coal particles, the faster the ignition of pulverized coal, resulting in improved temperature and increased area of slagging in the furnace.