Abstract: The sequential chemical extraction (three steps) was used to examine the modes of occurrence of alkali metals in four kinds of Xinjiang coals based on the solubility in distilled water, ammonium acetate and hydrochloric acid. The water-soluble anions were analysed by ion chromatography. The contents of alkali metals in the coal ash made at different temperatures and residence times were measured respectively, and the release forms of alkali metals from coal ash were also simulated by Factsage. Results show that the most of sodium in coal is the water-soluble one and the potassium only exists in the insoluble form. Water-soluble alkali metals may exist in the form of hydrated ion of chloride. The release of alkali metals from coal is the fastest during 400~600 ℃,which are mostly water-soluble alkali metals; and the release of alkali metals mostly occurs at the later stage of combustion. Alkali metals in ash would react with the components of flue gas at high temperature, which produces chloride and hydroxide. It can be inferred that the sodium makes a great contribution to the formation of low temperature eutectoid at 700 ℃.