Coal with a high mullite concentration in ash is generally believed to have a high ash fusion temperature (AFT) and low slagging tendency. Conducted by a mineral transition mechanism at high temperature condition, this presented work investigates the improvement of the coal ash slagging tendency by coal washing and additive blending with mullite generation. AFT is considered as the basic criterion for evaluating coal slagging tendency and tested by an intelligence ash melting point apparatus in the investigation. Low temperature plasma asher was used to manufacture ash samples containing original minerals. X-ray diffraction was used to detect mineral concentration in all ash samples. Four fusible coals with different mineral compositions and bauxite (as an additive) from Henan Province, China were chosen to be studied. On the basis of experimental results, this study shows that the slagging characteristics quantitatively depend on the mullite concentration in ash, which means that coal slagging can be significantly alleviated through coal washing and additive blending. Results show that coal washing removes large amounts of surface ash and impurities to avoid eutectics. Consequently, the mullite concentration in ash increases and the AFT significantly increases. Meanwhile, adjusting ash mineral composition twice by adding bauxite to coal after washing can also do favor to ash slagging tendency decreasing.