Abstract: Oil shale is characterized by a dense structure, a low proportion of pores and fissures, and low permeability. Pore-fracture systems serve as crucial channels for shale oil migration, directly influencing the production efficiency of shale oil resources. Effectively stimulating oil shale reservoirs remains a challenging and hot research topic. This investigation employed shale specimens obtained from the Longmaxi Formation. Scanning electron microscopy, fluid injection experiments, and fluid–structure interaction simulations were selected as experimental approaches. The structural changes and fluid flow behaviour of shale under high temperatures were comprehensively revealed from microscopic to macroscopic levels. The experimental results indicate that the temperature has little effect on the structure and permeability of shale before 300 °C. However, there are two threshold temperatures between 300 and 600 °C that have significant effects on the structure and permeability of oil shale. The first threshold temperature is between 300 and 400 °C, which makes the oil shale porosity, pore-fracture ratio, and permeability begin to increase. This is manifested by the decrease of micropores and mesopores, the increase of macropores, and the formation of a large number of isolated pores and fissures within the shale. The permeability increases but not significantly. The second threshold temperature is between 500 and 600 °C, which increases the permeability of oil shale significantly. During this stage, micropores and mesopores are further reduced, and macropores are significantly enlarged. A large number of connected and penetrated pores and fissures are formed. More and thicker streamlines appear inside the oil shale. The experimental results demonstrate that high temperatures significantly alter the microstructure and permeability of oil shale. At the same time, the experimental results can provide a reference for the research of in-situ heating techniques in oil shale reservoir transformation.