The unstable supply of renewable energy due to seasonal dependency contradicts the periodic energy demand. As hydrogen presents high energy density and flow mobility, and low solubility and residual saturation, underground hydrogen storage (UHS) is a promising solution of scalable energy storage to rebalance energy demand and supply. Depleted gas reservoirs (DGR) are a suitable option for UHS due to their demonstrated integrity. In this study, we developed a numerical simulation model based on Tough þ RealGasBrine (T þ RGB) simulator to evaluate the feasibility of UHS in DGR. We calibrated different Equation-of-State (EOS) models for modeling the phase behavior of hydrogen-included mixtures with experimental data, and this further helps to examine the impact of various cushion gas pre-injection strategies on pressure maintenance and potential hydrogen leakage through cap-rock. After comparison among three EOS models, we adopted the Soave-Redlich-Kwong (SRK) EOS in our simulations as its high accuracy and computational efficiency. Afterward, we simulated one hydrogen injection-idlewithdrawal operation in a high-resolution mesh based on a synthetic heterogeneous anticline DGR. We observed that hydrogen displaces pre-existing methane and resides at the top of the storage zone due to gravity segregation. The average pressure and gas saturation of the whole simulation domain increase with the hydrogen injection. With regard to the hydrogen leakage, when the cap-rock permeability ranges from 105 to 103 mD, 0.05% of the injected hydrogen at maximum leaks into the cap-rock while about 1% of injected hydrogen is dissolved into the aqueous phase. Those results demonstrate that DGR is a viable option for UHS. However, only about 73% of injected hydrogen can be recovered if the bottom-hole pressure of the production well is 2 MPa below the reservoir pressure. The cushion gas becomes necessary for the UHS project to increase hydrogen recovery. Injecting cushion gas of nitrogen and carbon dioxide increases hydrogen recovery from the initial 73%e91% and 81%, respectively. The simulation results demonstrate that nitrogen exhibits better performance, in terms of higher hydrogen recovery factor and purity of producing gas, as the cushion gas. In this work, we quantitatively evaluated the hydrogen leakage problem, including leakage into cap-rock, dissolution in water, and mixing with other gas components, which is the first-of-its-kind analysis in literature to the author's best knowledge. The simulation results support the feasibility of UHS in the DGR.