<p>Coal mining leads to the formation of caving zones, fracture zones and bending zones from bottom to top of the overburden. The mechanical compaction properties of the broken rock mass in the caving zone are completely different from those of the original rock and have a significant impact on the movement mechanism of the overlying strata. To study the influence of particle size and gradation on the mechanical compaction properties of broken rock in the caving zone, broken blocks with different particle sizes are made by physical modeling, and broken rock specimens with different characteristics are prepared for compaction testing according to uniform mixing and graded mixing. The results show that a small particle size is the main size controlling the mechanical compaction properties of broken rock masses. At the initial stage of compaction, the rotation, translation and sliding of broken rock blocks lead to the reduction of voids and specimen volume. The contact between them is mainly “point-to-point” and “point-to-face”; with the increase of axial stress, the contact gradually changes to “face-to-face”, and the small particles produced by grinding will fill the gap between blocks and strengthen the bearing capacity of the “face-to-face” contact of the broken rock mass. After compaction, the crushed rock mass with uniform particle size is mainly composed of rounded blocks and small particles generated by grinding, whereas the crushed rock mass with graded particle size is mainly composed of complete rounded blocks. The stress-strain curves of broken rock masses with uniform particle sizes and graded particle sizes can be well fitted by the Salamon hyperbolic model.</p>