The evolution of pore structure in oil shale under the influence of high-temperature fluids plays a decisive role in the effective in-situ heating and efficient production of pyrolysis oil and gas. This study conducted a series of supercritical carbon dioxide pyrolysis experiments on Longkou oil shale samples using high-temperature, high-pressure reactors. Through methods such as low-field nuclear magnetic resonance and high-pressure mercury intrusion, we systematically analyzed the changes in pore structure and connectivity before and after the pyrolysis of oil shale. The results show that as the temperature of supercritical carbon dioxide increases, the pore volume in the oil shale gradually increases with the temperature, and the connectivity of the pores also progressively strengthens. After pyrolysis, the distribution range of pore sizes in the oil shale broadens, and the number of large pores gradually increases. Furthermore, as the temperature rises, the organic matter in the oil shale undergoes more complete decomposition, accompanied by the release of internal gases. This release of gases increases the pressure within the pores, potentially leading to the formation of microcracks in the rock layer. These microcracks further facilitate the transformation of transitional pores into larger pores, a trend that continues to expand with increasing temperature. Moreover, the changes in pore structure are not only related to the pyrolysis process but also affect the migration patterns of the pyrolysis products.