Abstract: To elucidate the influence of the intrinsic properties of coal gasification slag on its compaction characteristics and thereby further increase the landfill capacity per unit volume, the fundamental properties and compaction characteristics of two coal gasification slag samples, namely fine slag and coarse slag, were systematically investigated. The fundamental properties included particle size distribution, microscopic morphology, specific surface area, and micropore volume. The median particle sizes (D50) of the fine and coarse slag were 80.08 and 510.08 μm, respectively, whereas the corresponding D90 values were 454.02 and 969.09 μm. The fine slag contained a greater proportion of loose and porous particles, whereas the coarse slag was characterized by a higher proportion of dense and irregular particles. Consequently, the fine slag exhibited a larger specific surface area and micropore volume than the coarse slag. Under an applied pressure of 1.6 MPa, the compacted densities of the dry fine and coarse slag samples reached 0.425 and 0.978 g/cm³, respectively. Following moisture addition, the maximum dry density of the compacted fine and coarse slag increased to 0.577 and 1.068 g/cm³, corresponding to increases of 35.7% and 9.2%, respectively. Under dry conditions, incorporating fine slag into coarse slag did not improve the compacted density of the blended material. In contrast, the removal of porous carbon particles from coal gasification slag through burning pretreatment significantly increased the compacted density of the dry samples for both slag types, with enhancement rates of 41.41% and 72.19%, respectively.