Abstract: Underground hydrogen storage (UHS) and carbon dioxide capture and storage (CCS) have been the frontiers of energy transition of petroleum and coal industries. A pore-scale network simulation method incorporating non-wetting phase hysteresis effects has been developed to study the flow characteristics of hydrogen and carbon dioxide in saline aquifers. Field-scale numerical simulations of hydrogen and carbon dioxide injection, as well as hydrogen recovery, were conducted to evaluate gas storage capacity, storage safety, and recovery efficiency. The results indicate that carbon dioxide has a higher likelihood of leakage compared to hydrogen during storage. When the storage capacity of the reservoir is limited, carbon dioxide storage is more advantageous than hydrogen. Pre-injecting carbon dioxide as buffer gas can effectively enhance hydrogen recovery efficiency, where the Pareto digram for optimal hydrogen storage efficiency is provided. This research outcome offers scientific guidance for both theoretical studies and engineering practices of hydrogen storage and carbon sequestration in saline formations.