The oil in place from Qiuling Oilfield in Tuha Basin is light. The heterogeneity of the reservoirs leads to low water flooding recovery，and the reservoir has entered the high water-cut stage. In view of the feasibility of gas miscible flooding after water flooding of the reservoir，the miscible pressure of the first-stage separator gas（PSN gas）and oil in place was tested by a slim tube experiment，and the type of miscible additives（liquefied petroleum gas（LPG），toluene，2-Butoxy ethanol，CO₂，and enriched natural gas）and the amount of slug injection were optimized. Then，the oil displacement effect of miscible additive slug flooding+PSN gas flooding+subsequent water flooding at the high water-cut stage was studied by using a long core model composed of formation core in the oilfield. The results of the slim tube experiment show that miscible flooding cannot be achieved through the displacement of oil in place by PSN gas under current formation conditions，and it is necessary to add a miscible additive slug in front of injected gas. Long core experiments show that gas flooding after water flooding can greatly enhance the recovery of oil in place. The oil displacement effect is greatly improved by adding a miscible additive slug in front of the injected gas，and the recovery increases with the increment of the injection amount in the miscible additive slug. However，the effect of miscible additives（CO₂ and enriched natural gas）with weak solubility in oil in place is different from that of miscible additives（LPG）with strong solubility，and higher injection volume may yield a worse effect.