Abstract: During the top coal mining process of fully mechanized caving in thick coal seams, the collapse of rock strata and the evolution of fractures in the goaf area significantly affect the gas migration path and enrichment characteristics, and thereby are related to the occurrence and prevention and control of gas disasters. To systematically reveal the mesostructure changes during the collapse process and their coupled response mechanism to gas migration, a three-dimensional mining model of thick coal seams was constructed based on the discrete element software PFC^3D, simulating the development of roof fractures and the evolution law of porosity during the advancement of the working face. Furthermore, a fluid domain proportional to the PFC model was established in combination with Fluent, and the porosity data was coupled and imported through UDF to construct a gas migration simulation model in the goaf. The results show that the roof collapse presents obvious phased and spatial heterogeneity, and the fractures expand vertically and form a delamination structure. The porosity distribution changes from loose to compacted with the advancement process, forming a dynamic evolution trend. The concentration of gas is jointly controlled by the collapse structure, porosity field and air leakage disturbance. High-concentration accumulation areas are prone to form in the return air corner and the deep part of the goaf, posing an explosion risk.