Aimed at the problem of mixing working gas and cushion gas in carbon sequestration technology, the feasibility of using cation dioxide as the cushion gas in reservoirs is discussed firstly. At the usual condition of reservoirs, carbon dioxide is a kind of supercritieal fluid with high condensability, high viscosity and high density. Secondly, this article studies the laws of formation and development of mixing zone by numerical simulation and analyses the impact on mixing zone brought by different injection modes and rational ratios of cushion gas in reservoirs. It is proposed that the appropriate injection ratio of cushion gas is 20% - 30%. Using carbon dioxide as cushion gas in gas reservoirs is able to make the running of natural gas reservoirs economical and efficient.