Underground gas storage (UGS) is a beneficial economic method of compensating for the imbalance between natural gas supply and demand, which is currently considered an important part of the gas chain. To reduce the capital investment of gas storage, the use of alternative gases as cushion gas is a good choice. However, the mixing of different gases in the UGS challenges the application of alternative gases(N2and CO2) as cushion gas. In this paper, we first conduct the thermodynamic analysis of two alternative gases, namely, N2and CO2. The feasibility of these two gases as cushion gas is discussed. Secondly, a gas storage located in Eastern China is selected. Based on the physical properties of the reservoir, a simulation model with coupled diffusion mechanism is conducted. Finally, the effect of molecular diffusion, gas category and ratio on the dynamic operation of UGS is studied. In addition, the CO2storage capacity under different operating parameters was also analyzed. This study provided a scientific basis for the efficient operation of UGS and the geological storage of CO2.