As CO₂ injection can enhance the efficiency of shale oil extraction and reduce CO₂ emissions, it has been utilized widely in the development of shale oil resources. Minimum miscible pressure is an important parameter describing the miscibility of CO₂ and shale oil, which is of great importance for determination of CO₂ injection strategy. However, due to the unclear phase boundary caused by the confinement effect in shale nanopores, it is difficult to determine the minimum miscible pressure of CO₂ and shale oil. In this study, a new minimum miscible pressure estimation method is constructed, that is suitable for nanopores based on the significant co-evolution of pore wall adsorption and confined-bulk phase interactions. This method can mitigate the limitations of traditional minimum miscible pressure calculation methods relying on fluid interfaces. Furthermore, the confinement effects on the miscibility process are analyzed using a theoretical method and molecular dynamics simulation on the microscopic scale. The results demonstrate that the minimum miscible pressure of CO₂ and shale oil initially decreases as the pore size decreases. When the pore size decreases to a certain extent, the minimum miscible pressure increases with the thickness of the adsorbent layer rising and the CO₂ diffusion coefficient decreasing. Temperature elevation raises the minimum miscible pressure as it intensifies molecular thermal motion, weakens fluid adsorption, and reduces interaction energy, which are not conducive to miscibility. This study can provide an essential basis for the optimization of CO₂ injection pressure in shale oil reservoir development.