Aimed at the problem that it is difficult to simulate the gravity segregation in real reservoir condition by indoor experiments，the gravity segregation physical model in high temperature and high- pressure gas flooding that can stimulated the gravity segregation under reservoir conditions was developed independently. the gravity segregation physical model in high temperature and high pressure gas flooding that can stimulated the gravity segregation under reservoir conditions was developed and gravity segregation was characterized by the produced fluid difference of upper and lower layers，CO2 immiscible flooding using long cores was conducted to study gravity segregation at different injection rates，the gravity segregation degree was evaluated through oil recovery proportion at upper and lower layers of core when gas breakthrough，final recovery and core displacement profiles，and the mechanism that how gas injection rate affected gravity segregation was also analyzed. The results showed that，the gravity segregation was serious that oil recovery of lower layer was nearly zero and oil recovery of top reached up to 97.5% when gas injection rate was at low value of 0.1 mL/min，resulting in the whole sweep efficiency of core was so poor that final oil recovery was only 49.49%. The oil recovery proportion of upper layer reduced to 50.9% as gas injection rate increased from 0.1 mL/min to 1 mL/min，and the final oil recovery even reached up to 60.26%. The residual oil of core displacement profiles decreased since gas injection rate increased，indicated that the gravity segregation degree got weaken. With increasing gas injection rate，CO2 level viscous force increased，the suppression of vertical gravity caused by oil- gas density difference was enhanced，the core sweep efficiency was improved as gravity segregation declined. Experimental results can provide theoretical basis for optimization of CO2 flooding gas injection parameters in low permeability reservoirs.