Injecting CO₂ into shale oil reservoirs has the dual purpose of enhancing oil recovery rate and geological storage of CO₂. The phase behavior and mass transfer laws of the CO₂-shale oil are crucial for achieving highly efficient production shale oil and gas. Based on an improved vapor-liquid equilibrium (VLE) model in nanopores, which achieves a relative error of less than 3.16%, this study investigates the phase behavior and CO₂-oil mass transfer laws of two representative shale oils under CO₂ injection. Research results show that after CO₂ injection, the saturation pressure tends first to increase and then reduce. The CO₂ dissolution results in a significant reduction in the molecular weight and viscosity of Wolfcamp shale oil. During the multi-contact shale oil with CO₂ injection, both density and the interfacial tension (IFT) reduce at the forward contact edge, indicating forward-contact miscibility. Furthermore, the nanoconfinement effect reduces the density difference between the liquid and gas phases, leading to a reduction in the IFT, which is beneficial for gas-oil miscibility. The study results can provide a theoretical basis for understanding compositional variations in shale oils under CO₂ injection, offering promising improvements in miscibility and recovery efficiency.