To study the dynamic water storage performance and evolution mechanism of fractured rock mass in the caving zone under overburden pressure, lateral compaction tests were performed on fractured rock samples of coarse sandstone, sandy mudstone, and mudstone with a self-developed visualization device. The porosity, bulking coefficient, and spatial evolution images of the entire compaction process were obtained. The experimental results indicated that the compaction process of fractured rock mass can be divided into four stages: large gap compaction, small gap compaction, post failure compaction, and compaction strengthening. Among them, the changes in water storage space of rock mass mainly occur in the first and third stages. Due to differences in rock mass strength and water softening characteristics, the three types of fractured rock contain different compaction processes and segmented characteristics, resulting in water storage spaces in sandy mudstone being 3.75 and 7.5 times larger than those in coarse sandstone and mudstone, respectively. As particle size increases, the stability of the fractured rock mass improves, with porosity increaasing by 0.0,0.09, and 0.04 for coarse sandstone, sandy mudstone, and mudstone, respectively. The experimental results can be well explained by particle mechanics, where the compaction process of rock mass is essentially characterized by the formation of slip, compression, and force chains among loose rock blocks. Variations in lithology, overburden pressure, and particle size distribution result in differences in the order and process of occurrence of the three stages, which in turn leads to differences in the water storage characteristic of the rock mass. The research results can provide theoretical support for the accurate evaluation of water storage capacity of underground reservoirs in coal mines.