<p>Clarifying the compaction characteristics of broken roof rock (BRR) in deep goafs is essential for predicting surface subsidence and the porosity distribution in goafs caused by deep coal mining. For deep core samples from the Pingdingshan mining area, basic mechanical tests and compaction tests on graded BRR were conducted for four rock types, and the effects of lithology and particle size on BRR compaction behavior were analyzed. The results show that the initial bulking coefficient of BRR generally increases with particle size, and the growth trend is governed by rock compactness; the initial bulking coefficient of mixed-grade gravels lies between those of the individual particle-size groups. The residual bulking coefficient is more strongly influenced by lithology than by particle size. During compaction, the gravel exhibits three stages: rapid compaction, slow compaction, and compaction stabilization. The stress–strain response in different stages is jointly controlled by lithology, particle size, and gradation. With increasing compaction stress, the porosity of the gravel decreases exponentially, and the average porosity after compaction, from high to low, is sandstone, limestone, sandy mudstone, and mudstone. This study provides a theoretical basis for understanding porosity evolution in deep goafs, gas drainage, and overlying strata subsidence prediction.</p>