The reducing atmosphere at these regions between the top burner and the SOFA nozzle becomes stronger after the low-NO_x combustion retrofit for a coal-fired boiler. Under this condition，the risk in high-temperature corrosion on water-walls increases sharply. Taking a 330 MW Tangentially Pulverized coal boiler as the object，samples were collected from water-walls surface at different position. The ash and corrosion layer were characterized by elements and minerals，and the causes of slagging on the water-cooled wall in the furnace and the high-temperature corrosion mechanism of pipe wall were analyzed. The results show that Si and Al elements in slags adhered to the water wall at different positions mainly exist as Al₆Si₂O₁₃ and Al₂SiO₅，which increase the fusion temperature of ash particles. Fe element mainly exists in Fe₂O₃ and its enrichment is significantly higher than alkali or alkali earth metals. S and Zn elements and a small amount of Pb element are highly enriched in the layered deposits on water-walls which are located at the central height of B and F layer of burner. The content of Zn element is as high as 20%，mainly in the form of PbS，ZnS and ZnAl1.04S2.13. The fly ashes deposited on the surface of slags collected at the central height of F layer burner still contains about 7% unburnt carbon，which indicates that swirling flue gas may scour the water-walls. These fly ashes are mainly composed of Si，Al，Fe，C，S and Zn elements，mainly in the form of mullite，hematite and sulfides. The corrosive layer on the water-wall is mainly Fe₁-xS，Fe₇S₈，Fe₉S₁₀ and Fe₃O₄，and contains a small amount of PbS and PbO1.57. The formation of Fe1-xS with varied crystal structure may be attributed to the difference in the local H2S partial pressure and temperature. Zn and Pb containing species are enriched in layered deposits via vaporization-condensation and the release from unburnt carbon particles adhered on depositing surfaces. While silicoaluminate-rich and iron-rich ash particles stick to the surfaces of water-walls via inertial impaction and thermophoresis deposition.