High-alkali species in coal are notorious for causing ash slagging and fouling incidents. In this paper, four high-alkali coals were individually subject to hydrothermal pretreatment（HTP）, within a batch-type autoclave at 300 °C for 1 h, and the treated coals were analyzed, along with the oxygen-containing functional groups determined by Fourier transform infrared spectrometer（FT-IR）. Then the alkali species and other components in the coal ash were quantified by X-ray fluorescence（XRF） for evaluating the ash slagging and fouling tendency. Apart from this, FactSage was adopted to simulate the occurrence and transformation of alkali species during coal thermal conversion ending at various temperatures. The findings indicate that the treated coals are superior to the parent ones in terms of certain remarkable changes via HTP. The moisture, oxygen and sulfur of the hydrothermally treated coals decline obviously, while the calorific value rises sharply. HTP could reduce the alkali species to less than 2%（%, by weight, equivalent to Na2 O in dry ash）, with a maximum removal ratio of 88.9%, lowering the ash slagging and fouling tendency. The proposed mechanism of HTP was that the alkali species in coal matrix became released due to the breakage of the coal functional groups and micropores during HTP.
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