Abstract: Gas and liquid flows coexist in deep-buried high-pressure low-permeability reservoirs after natural depletion. Further studies are demanded due to the complicated characteristics of CO₂ miscible flooding. The environment of deep-buried high-pressure low-permeability reservoir was simulated and the laboratory experiment of CO₂ flooding was carried out using slim tube and long core. It has been concluded that after natural depletion, gas and liquid flows coexist in formation. The minimum miscible pressure (MMP) and the effect of CO₂ flooding decline. But after restoring formation pressure, the effect of CO₂ flooding increases, and achieves the similar result to that by continuous CO₂ injecting under original formation pressure. The conclusion has been applied in CO₂ flooding pilot test of deep-buried high-pressure low-permeability reservoirs in block Hu96 in the Zhongyuan Oilfield, providing technical support for the development of deep-buried high-pressure low-permeability reservoirs.