Abstract: Borax fluxing agent was added to four kinds of coal ash in terms of different quality ratios. The ash fusion test (AFT) of the mixtures of coal ash plus borax fluxing agent was carried out and the reactivity of mullite in coal ash was calculated by the density functional theory (DFT). The space group of the mullite is Pbam, belonging to the orthorhombic system. The results suggest that mullite is likely to combine with electron acceptor rather than with electron donor. The activity of Si is higher than that of Al when mullite combines with the electron donor. When borax is added into mullite, Na+ as the electron acceptor easily enters into the lattice of mullite, causing a transition from mullite to nepheline. The calculation results agreed well with the AFT results under reducing atmosphere.