Abstract: A 1000 MW ultra-supercritical once-through boiler burning high-sodium, low-melting-point Indonesian coal has experienced severe slagging and fouling, particularly in the low-temperature superheater and reheater regions. To investigate the mechanisms responsible for these issues, this study conducted a detailed physical and chemical analysis of slagging and fouling samples collected from the furnace, platen superheater, high-temperature reheater, and low-temperature superheater and reheater areas. The results suggest that one of the primary causes of severe slagging and corrosion when burning Indonesian coal with a low ash fusion temperature is its relatively low ash melting point. At high furnace temperatures, the ash becomes molten and adheres to the screen tubes, forming small but hard slagging layers. This phenomenon is mainly observed in the furnace and platen superheater areas, where slagging, although not severe, causes significant deterioration in heat transfer. Another contributing factor is the presence of alkali metals in the coal ash, which mostly exist as low-melting-point sulfates. These sulfates vaporize easily in the high-temperature furnace environment and subsequently condense onto heat exchange surfaces, forming a highly adhesive fouling layer. This layer then captures fly ash particles, leading to slag formation. This issue is most prominent in the low-temperature reheater and superheater areas. By analyzing the slagging and fouling mechanisms in various heat exchange regions, this study provides valuable insights and recommendations for the future utilization in ultra-supercritical once-through boilers.