Abstract: Deposition or slagging problem caused by the alkali metal species is one of the major obstacles to utilize biomass fuel in combustion and gasification plant. The paper mainly studies the effect of water leaching on alkali-induced slagging properties of corn straw and rice straw. The original mineral form of alkali elements in biomass straw was studied with comparison of low temperature ash of biomass before and after water leaching. The release and transformation of alkali compounds in biomass straw at different temperatures during heating were analyzed with combination of the XRD result of the heated biomass and the chemical composition of the digested samples. The result shows that the potassium in corn and rice straw mainly exists in the form of KNO₃, KClO₄, K₂SO₄ and KAlSi₃O₈, in which KNO₃, K₂SO₄ and KClO₄ is mostly removed in water leaching. The fusion temperatures increased after water leaching, especially for rice straw ash, which is a typical sample that the basic compounds are mainly potassium-containing compounds. The decrease of potassium content with temperature in the original corn and rice was because the potassium-containing minerals decomposed and escaped at 25−1000 °C. The release of potassium in the range of 400−800 °C significantly reduced for corn and rice straw after water leaching, but the potassium content will still decrease above 800 °C. The decrease of magnesium content in corn with increasing temperature may be due to the volatilization of magnesium oxide under the action of carbon reduction. For fuels with high alkali metal content after water leaching, the residual alkali metal will still escape and cause deposition or slagging in the furnace, especially in the temperature range above 800 °C.