Numerical studies of the slagging characteristics under different operational conditions in a 300 MW down-fired boiler were carried out using slagging models coupled with gas-solid two phase flow and combustion models. Combined with the real operating conditions; comparative and detailed analysis on the slagging position, extent, and causes is presented. The results show that the serious slagging is mainly on the side walls of the lower furnace. Because of the more rapid expansion of the flue gas under the higher temperature, the flue gas in the furnace center makes the flue gas on both sides deflect and flow to the side walls: and the pulverized-coal flame impinges on the side walls. This results in the slagging on the side walls. Under off-design operating conditions, such as stopping some burners, the local flow field is asymmetric and impinges on the local arch burner, front and rear wall regions where the stopped burners are located. It leads to slight slagging on the arch burner regions and the front and rear wall regions of the lower furnace. Based on the investigation, it has been found that the serious slagging on the side walls can be effectively alleviated by cutting off the burners close to the side walls, reducing boiler load and burning low slagging-tendency coals. (C) 2009 Elsevier B.V. All rights reserved.