Abstract: Zhundong coal blending was combusted in a 350 MW boiler. Ash deposits samples at different heat transfer surface (HTS) were collected to investigate mineral conversion of each sample. Thermodynamic equilibrium calculation was carried out to give theoretical support by virtue of Factsage 5.2. Different mechanism of each HTS was concluded as follows. At high temperature HTS, albite, anorthite and other phases help eutectics precipitate take shape, which adheres to fly ash particles and forms indurate bulk ash deposits. At low temperature HTS, sulphur-containing species are condensed from flue gas to form deposits with evidence that the main crystalline phase is anhydrite. At economizer HTS, The deposits consisted of amorphous phases carried by flue gas are loosely bonded. Calculated results by Factsage basically agreed with the phase composition of actual samples and are of assistance to mineral conversion analyses.