Supercritical carbon dioxide (SC-CO₂) has been progressively used in the development of shale oil and gas. However, the interaction between CO₂ and shale can change the mineral composition and the pore structure, thus affecting the mechanical properties of shale. To study the influence of SC-CO₂ on shale, shale samples collected from the Songliao Basin in China are treated with SC-CO₂ at various time intervals. Then, a series of tests are performed, such as the mineral composition analysis test, the pore structure analysis test, and the macro mechanics test. The results show that the mechanical properties of shale gradually decrease exponentially with the increase of SC-CO₂ treatment time. The loss of elastic modulus reaches about 47% after the treatment of 14 d. X-ray diffraction analysis shows that the mineral (except quartz) content decreases after SC-CO₂ treatment, and in particular, the proportions of carbonate minerals significantly decrease by about 12%. The primary pores and fractures are eroded through dissolution, and new pores and fracture structures are developed on the surface microstructure. In addition, the proportion of micropores and mesopores decreases, while the proportion of macropores increases after SC-CO₂ treatment. The specific surface area and average pore size present upward trends during SC-CO₂ treatment. The changes of mineral assemblage and pore structure lead to the obvious decline of mechanical properties in shale reservoirs. This study contributes to understanding the evolution characteristics of mechanical properties under SC-CO₂ treatment, which is of great significance for the efficient exploitation in shale reservoirs.