Abstract: In the process of coal seam mining, how to achieve large-scale disposal of coal gangue has always been an important problem to be solved in the green and efficient development of the coal industry. As the main space for gangue grouting and filling, the evolution process of mining voids in the caving zone is closely related to the movement of overburden rock. By analyzing the movement law of overlying strata in the caving zone, it is found that the fillable space of the caving zone is one of the fundamental ways to solve the above problems. Taking the 122109 ultra-large height working face of Caojiatan Coal Mine as the research object, a method combining similarity simulation experiment and theoretical analysis was adopted to study the morphological characteristics of the caving zone, the subsidence law of the overlying strata, and the fillable space of the caving zone. The evolution law of the overlying strata moving from bottom to top gradually was revealed. Based on this, the images were processed using the ImageJ software, and the fractal dimension of the fracture network diagram was calculated. This quantitatively characterized the development degree and complexity of the fractures. The results show that under the working face mining conditions, the development of overlying rock fractures in the caving zone can be divided into stages such as the stage of fracture development, the stage of accelerated fracture development, the stage of compaction, and the stage of overall stable fracture; the proportion of void area in the caving zone is 28.6%, and the void space on the side of the transportation roadway of the “O-shape ring”, the bed separation zone between the caving zone and the fractured zone, and the void space on the side of the return air roadway of the “O-shape ring” and the void space within the caving zone account for 14.6%, 14.0%, 15.1%, and 56.3% respectively. Based on the void calculation model of the loose rocks in the caving zone, the void ratio of loose rocks in the caving zone is 29.13%, the available filling space of the caving zone is 5 366 000 m³, the available filling volume of the coal gangue on one side of the caving zone is 2 065 000 t, and the filling rate is 9.4%.