Two typical kinds of coal respectively with high and low ash fusion point were taken as the object of study, corresponding ash fusion behavior and related mineral evolution mechanism under hightemperature gasification conditions were analyzed using XRD and SEMEDX method, and calculated with quantum chemistry method. Results show that the high ash fusion temperature (AFT) of ash A is resulted from the formation of large amount of mullite at high temperatures; whereas the low AFT of ash B is caused by fluxing minerals such as anhydrite and soda feldspar etc. existing in ash B, and also by the chemical reaction of CaO with mullite to form anorthite and gehlenite during ash melting process. Since mullite is likely to combine with electron acceptors rather than with electron donors, it easily reacts with metal cations such as Ca2+, Mg2+, Fe2+, Na+ and K+ etc., to generate some other minerals, therefore, the ash fusion behavior can be controlled through adding different kinds of metal cations to change the AFT of coal ash.