||Because of adopting a high air-staging combustion ratio for Chinese coal-fired boilers, the water walls currently suffer from severe sulfidic corrosion and slagging. This work aims to understand the real deposition of ash particles and the high-temperature corrosion behavior at fireside, after making detailed characterizations for four types of deposits or slags and a corroded water-wall tube, which were collected from a real coal-fired boiler. The profile section of the corroded tube presents three layers with respective features. The major phases for the manually peeled layer from the corroded tube are identified as polymorphous pyrrhotite (Fe7S8, Fe9S10, Fe1–xS), galena, and lead oxides. Above the corroded layer the deposits are mainly composed of ZnS- and Si–Al-rich phases, minor of Fe-rich particles. This is mainly attributed to the direct condensation of gaseous PbS or Pb, the sulfidation of gaseous Zn and Pb, and the thermophoretic deposition of ZnS-/Si–Al-rich fine particles. The impact and erosion of particles promote the splitting of the outer corrosion layer into small parts and then peeling off from the inner layer. Additionally, enrichments of various trace elements including As, Ge, Ga, Th, and Sn are attributed to the direct condensation of metal vapors and their revaporization–condensation from the unburnt particles. The oxidation and sulfidation of iron are competitive and responsible for continuous corrosion. Although Fe sulfidation can be suppressed in the regions in which either ZnO or PbO is stable, Fe can be sulfided or oxidized in the regions where ZnS or PbS is stable.