The molecular adsorption in nano-scale shale pores results in the reduction of effective pore volume and further strengthens the confinement effect. This study aims at examining the adsorption effect coupled with confinement effects on phase behaviors of shale fluids. First, a modified extended Langmuir formula was developed to calculate the adsorption amount for a multi-component shale mixture. A modified cubic Peng–Robinson equation of state was proposed, and the occupied volume by the adsorbed phase was taken into account. The saturation pressures and fluid properties under the confinement effects and adsorption isotherms were examined. In order to examine the change of phase properties during a gas injection process in a shale condensate reservoir, we gradually increase the mole fractions of N2 or CO2 in shale condensate mixtures by coupling with confinement effects. We found that the thickness of the adsorption film reduces the effective pore throat, leading to intensified confinement effects and smaller bubble point pressures. When the gas adsorption layer is considered, a more significant decrease in density and viscosity is observed. The critical pressure of the condensate fluids increases and the critical temperature decreases with the continuous N2 injection. Contrary to N2 injection, the critical pressure decreases and the critical temperature moves upwards with CO2 injection. For condensate that accumulates in nano-pores (e.g., r ≤ 6 nm), the condensate fluid always exists in gas status during the gas injection and the subsequent production processes.