Knowledge of the effect of aluminosilicates in fly ash on slagging is very important for the prevention and control of slagging in coal-fired boilers. To understand hydrochloric acid dissolution of the aluminum element of fly ash aluminosilicates at a chemical coordination level and establish a reliable method to characterize fly ash aluminosilicates that easily cause slagging, we performed hot hydrochloric acid separation for fly ashes with different melting points obtained from four power plants in China and measured the acid-solubility of major elements in the fly ashes. The fly ashes and their acid separation residues were analyzed using X-ray diffraction and Al-27 magic-angle spinning nuclear magnetic resonance (Al-27 MAS NMR). The results show that the 6-fold coordinated aluminum (denoted as Al(VI)) in the fly ashes does not easily dissolve in hot hydrochloric acid. One part of the 4-fold coordinated aluminum (denoted as Al(IV)) easily dissolves in hydrochloric acid. The acid-soluble Al(W) in ash has stronger quardapolar interactions than the Al(IV) of mullite, and the excess negative charges of the AlO4 tetrahedron are compensated by active metal cations. The aluminosilicates containing a large amount of acid-soluble Al(IV) are easily slagged. In the fly ashes with high melting point, the mullite and corundum contents are high; the Al(W) content is low, and the acid-soluble Al(IV) is minor. In the fly ashes with low melting point, the aluminosilicates are mainly amorphous substances; the Al(W) content is high, and the acid-soluble Al(W) content is more. Therefore, the acid-soluble aluminum fraction of fly ash can be used to characterize the slagging propensity of aluminosilicates in fly ash.