Abstract: The recovery percent of reserves by steam flooding in the heavy oil reservoirs of the upper Qigu formation in District 9-6 of Karamay oilfield is 60%, but the recovery percent of reserves of the lower Qigu formation is only 32%. New development methods are needed to solve the problems of partial unswept reservoirs and large differences in reservoir producing degree, and further to improve development effect. Based on the indoor physical simulation experiments, this article studies the steam-CO2 flooding effect and oil displacement mechanism in District 9-6 and optimizes the injection parameters of the combined flooding. The experiment results show that CO2 can open the flowing channel for steam flooding and reduce the steam injection pressure, meanwhile it can inhibit the forming of the high-viscosity water-in-oil emulsion resulted from steam flooding and reduce crude oil viscosity; the water from steam condensation forms a water-alternating-gas slug, which can slow down the CO2 channeling and increase the swept volume. The enhanced oil recovery of steam-CO2 flooding is 35.0% higher than that of steam flooding and it is proved that the steam-CO2 alternating injection is the best injection mode. When the steam temperature is at 220 ℃ and the steam-CO2 volume ratio ranges from 10：1 to 25：1, the final oil displacement efficiency can reach 80%