Abstract: Slagging is a serious problem limiting the large - scale combustion and utilization of coal ashes with high alkali. Two typesof combustion boiler slags with a high alkali metal and high alkaline earth metals respectively were used to investigate the slagging tenden⁃cy and slagging process by ash fusion temperature tester, thermomechanical analyzer, thermogravimetric-differential scanning calorimeter,XRD, and thermodynamic software FactSage. It is found that the alkali metal in the high alkali ash slag mainly exists in silicoaluminate,while the alkaline earth metal mainly exists in sulfate. The initial sintering temperature of high alkali coal ash slags (SH and PG) is higherthan the coal ash slags with high alkaline elements (GJ and CQ), but the ash fusion temperature range of GJ and CQ is larger than SHand PG. Both initial sintering temperature and deposition index can be used to evaluate the slagging tendency of coal ashes with high basicoxides, but there is no direct relationship between the slagging tendency discrimination index and the chemical composition of the ashCaO, K2O+Na2O, SiO2 / Al2O3, alkali acid ratio B/ A, and the content of alkaline earth metal calcium sulfate and alkaline metal silicateAluminate in the mineral composition. The melting of coal ashes with high basic oxides mainly passed through two stages. The first stageis connected to the melting of low-melting-point alkali metal minerals (albite, nepheline) or the reaction with other minerals in coal ash(quartz, albite) to form the initial liquid phase, the second stage is mainly dominated by low-temperature eutectics which are formed bypyroxene and feldspar.