Block S of A oilfield is a typical ultra-low permeability reservoir. The conventional waterflood development method has some problems, such as water injection difficulty and serious underground energy deficit. Currently, the development of water drive to carbon dioxide drive is being carried out. In CO2 flooding and oil recovery, the water-gas alternating injection technology can effectively reduce the flow ability of CO2 gas with strong flow ability in the reservoir flow, effectively control the displacement front mobility, inhibit gas channeling and prolong the gas discovery time, so as to improve the CO2 flooding effect. In this paper, the influence of alternating injection technology of water and gas on the CO2 displacement effect was studied by analyzing the CO2 displacement situation in block S and the gas characteristics, and the influence of injection parameters such as gas injection speed, slug size and gas-water ratio on the oil stability and gas control effect of CO2 displacement was discussed.