Abstract: The reservoir in the small well spacing CO2 flooding test area of the Block Hei 79, Jilin Oilfield is featured with serious heterogeneity, unbalanced displacement plane, poor gas entry and water injection profiles and gas channeling, which influence the recovery factor of miscible flooding. Due to the high reservoir temperature (96.7 ℃) and the average reservoir pressure reaching 23.9 MPa in the test area, the stability of the conventional CO2 foam system is poor. The paper proposes a nano-particle/ CO2 foam system under high-temperature high-pressure reservoir conditions and performs evaluation for its application performance under simulated reservoir conditions. The experiment results show that under the reservoir conditions, the nano-particle/CO2 foam system has very good temperature resistance and salt tolerance, and with the increase of the reservoir pressure, the CO2 foam is easy to mix with foaming agent solution and a tighter net-like structure forms when it reaches a critical state, whose performance is better than that under normal pressure. The shunt rates of both high-permeability and low-permeability intervals are about 50% after injecting the nano-particle/CO2 foam system. During the displacement foam can effectively block high-permeability interval and modify gas entry profile. Finally the recovery factor of low-permeability interval can be improved