Fly ash containing high levels of alkali metals likely causes deleterious effects during municipal solid waste incineration, including fouling, slagging, corrosion, and deterioration of materials. This study aimed to understand the occurrence of alkali metals in fly ash, the attachment phenomenon of fly ash, and the corrosion mechanisms of high-temperature surfaces in heat exchangers. The presence of alkali metals was studied through a three-step extraction. Additionally, scanning electron microscopy energy-dispersive spectrometry was used to analyze the surface microstructure of fly ashes. X-ray fluorescence and X-ray diffraction (XRD) spectroscopies were used for content and crystalline phase analyses of alkali metals. The experimental results indicated that the surfaces of fly ash that contained higher levels of alkali metals were smoother and the content of alkali metals tended to increase with a slight fluctuation with the declining effluent gas temperature. The alkali metals were mainly water-soluble. On the basis of the XRD analyses, the water-soluble alkali metals were primarily alkali chlorides and sulfates, existing as NaCl, KCl, and K3Na(SO4)(2). Moreover, the transformation of alkali metals could accelerate corrosion of the heat exchanger as a result of the formation of a low-temperature eutectic melt.