The Chang 6 reservoir of Wangyao block in Ansai oilfield belongs to extra-low permeability formation, and prominent problems exist during the current water flooding. In order to enhance the oil recovery and evaluate the effect of CO2 flooding, optimization design of injection and production parameters for water alternating gas flooding is conducted with reservoir numerical simulation method. The PVTi module of Eclipse is used to simulate the gas-injection expansion experiment to analyze the effect of injected CO2 on the phase behavior of crude oil. With the orthogonal experiment design, four injection and production parameters are studied, including gas-oil ratio, CO2 injection rate, shut-in gas oil ratio and CO2 slug size. Visual analysis method is applied to analyze the test result. The results show that when the mole fraction of the dissolved CO2 reaches 60%, the viscosity of the oil reduces by 80.51% and the coeffi?cient of volumetric expansion reaches 1.44. The optimal combination of parameters is that the gas-oil ratio is 1∶1, gas injection rate at 13 000 m3/d, shut-in gas oil ratio at 1 000 m3/m3 and CO2 slug size at 0.005 pore volume.