Abstract: Aiming at the existing problem of void distribution in longwall goaf, through the similar simulation and theoretical analysis, the effect of mining thickness on the distribution of mining-induced pores in compression and tension zones of the longwall old goaf is obtained when mining thickness is 6 m, 8 m and 12 m respectively. When mining thickness increased from 6 m to 8 m to 12 m, fracture expansion coefficient and porosity of mining-induced fractures in collapse zone of compression zone increases first and then decreases with mining thickness, but overall, the fracture expansion coefficient tended to be 1.1, and the porosity tended to be 0.1. The porosity of compression fissure zone is close to 0.05, there are not only cracks but also holes in the tensile zone, and with the increase of the mining thickness, the volume of the hole increases first and then decreases. With the increase of mining thickness, the number of holes and porosity in the fracture angle extension zone increases obviously. Only when the mining thickness reaches a certain level, when the water-conducting fracture zone develops to the bottom of the key layer and stagnates, the separation hole can form. Finally, the correlation between the porosity or void volume of the tensile zone of the old longwall goaf and the mining height of coal seam, the height from coal seam, the comprehensive coefficient of crushing and swelling, and the deflection of the key layer is deduced.