Natural gas is a kind of national strategic resource, so underground gas storage is significant for the guarantee of national economic development and national energy security.Using CO2as cushion gas of gas storage is beneficial to CO2geological sequestration, enhance natural gas recovery and conserve resources. But the mixture of CO2and natural gas is inevitable.In this study, the inert gas mixing problem is investigated in storage reservoirs by using a reservoir simulator based on the equation of state. The degree of mixing between the two dissimilar gases was found by the amount of inert cushion that is produced along with the withdrawn gas. Reservoir parameters like the volume of inert, porosity, permeability, pressure and temperature were varied to study their effect on mixing. This paper studies the sensitivity of the mixing between CO2and natural gas by numerical simulation, finds out the main factors influencing the mixing, proves the great influence of dynamic injection-production on mixing. It’s shown that it’s better to inject cushion gas from the side of reservoirs.