Abstract: The molecular dynamics simulation, thermal dynamic calculation and experimental investigation were combined to illustrate ash slag viscosity variation mechanism for Al₂O₃-SiO₂-CaO-FeO system. The viscosity declines and the viscosity curve is transformed from crystalline slag to glassy slag with increasing mass ratio (C/F) of calcium to ferrous oxide in Al₂O₃-SiO₂-CaO-FeO system. There is an inflexion point when the C/F is equal to 2. When the C/F is below 2, there are mainly crystalline minerals in the system. While the C/F is above 2, there are mainly amorphous minerals in the system. With the increase of C/F, six-coordinated Al (AlO₆^9-) is transformed to four coordinated Al(AlO₄^5-) microscopically. Besides, the content of bridging oxygen decreases while that of non-bridging oxygen increases. Quantified function between base composition and viscosity are constructed based on the stability coefficients defined by oxygen bond species.