Abstract: To investigate the coal black liquor slurry′s ash deposition mechanism, a new method was proposed by contrast with coal water slurry. Ash particles were extracted from different locations of pilot boiler along the flue gas, in combination with sampling ash residue deposited on fire walls of the same distance from burner. The migrating tendency and deposition characteristics of main mineral elements along the flue gas were investigated in detail. In addition, XRD and SEM were carried out for coal water slurry and coal black liquor slurry′s ash deposit to get validation from microscopic view. The results indicate that the principal mineral elements causing ash deposition of coal black liquor slurry are sodium, silicon and aluminum. The ash deposit is composed of some phases at low fusing temperature, such as thenardite and nepheline. The minor factor is the migration and deposition of Fe, which is different from coal water slurry and coal.