Abstract: The composition and reaction of mineral matters in rice straw and corn stalk were investigated in weak reducing atmosphere at temperature range of 550-1300℃. The FactSage was also used to simulate the evolution behaviors of mineral matters under high temperatures. The results show that the existing form of Na and K in biomass is relatively stable, including volatilized chlorides, silicates and sulphates with a lower melting point. The silicates made up of Si, Ca, Mg, Fe and Al have various forms, mainly including pyroxene, olivine and melilite. Pyroxene is transformed into olivine and melilite with the increasing of temperature. The interaction of the three kinds of materials could result in the generation of eutectics which could lead to the melting of mineral matters in biomass. The formation of cristobalite and mullite leads to a higher flow temperature of rice straw ash than corn stalk ash, the mullite ultimately is transformed into low melting point materials like clinoferrosilite, iron spinel and anorthite.