The types of reservoir spaces in the fracture-cave carbonate reservoir are various and its shapes are complex. Attic oil and bypassing oil are easily formed during the process of water flooding. Gas flooding can effectively supplement formation energy and improve the recovery of remaining oil. In order to observe the displacement performance of injected gas in the fracture-cave reservoir，a two-dimensional visual physical model of fracture-cave reservoir is designed and made according to the actual geological data and injection-production characteristics of Tahe Oilfield as well as similarity principle.At the same time，the experimental study on enhanced oil recovery by gas injection in the karst-reservoir model of fracturecave reservoir is carried out. The results show that the macroscopic characteristics of oil-water interface are basically similar after displacement at 20 and 5 mL/min of gas injection rate，and there are obvious gas-water coning phenomenon and gas-water synergistic effect during nitrogen flooding. The ultimate recovery is about 70.5% at 20 mL/min of gas injection rate，and it is 78.9% at 5 mL/min of gas injection rate. High gas injection rate easily causes gas breakthrough. Low gas injection rate can prolong the time of nitrogen injection，so that gas breakthrough is not easy to occur prematurely. However，nitrogen can only enter caves and fractures with relatively low resistance due to low injection rate and low driving energy.Therefore，it is found that reasonable gas injection rate can enhance the potential of gas flooding in improving oil recovery.