Abstract: Objective With the increasingly urgent demand for large-scale hydrogen storage and transportation, Underground Hydrogen Storage (UHS) has emerged as a research hotspot due to its large storage capacity and cost-effective infrastructure. However, the competitive adsorption mechanisms between hydrogen (H₂) and cushion gases within clay mineral pores remain inadequately elucidated. This study aims to deeply clarify the adsorption mechanisms of hydrogen and cushion gases in caprock clay mineral pores at the molecular level, providing a theoretical basis for optimizing UHS operation strategies and cushion gas selection. Methods This study employed the Grand Canonical Monte Carlo (GCMC) simulation method to construct a slit pore model of Na-montmorillonite, with the goal of revealing the competitive adsorption mechanism between hydrogen and cushion gases in clay mineral pores from a molecular perspective. We systematically investigated the adsorption laws under various conditions of temperature, pressure, pore size, different cushion gases, and mixing ratios, based on hydrogen adsorption isotherms, molecular density distributions, and adsorption selectivity coefficients. Results (1) The adsorption capacity of pure hydrogen decreases with increasing temperature while increasing with rising pressure, exhibiting no significant variation within the pore size range of 2-6.5 nm. (2) Cushion gases significantly inhibit hydrogen adsorption capacity, with nitrogen (N₂) exhibiting particularly pronounced suppression; specifically, an adsorption plateau for hydrogen was observed at 6 MPa in the H₂-N₂ mixture system (60％:40％). (3) The selectivity coefficients of methane (CH₄) and nitrogen (N₂) over hydrogen consistently exceed 1, yet display a decreasing trend with increasing pressure, with the selectivity coefficient of nitrogen declining at a more rapid rate. Conclusion UHS reservoirs should implement high-temperature and high-pressure operational strategies. From the perspective of inhibiting hydrogen adsorption, nitrogen offers superior performance as a cushion gas compared to methane. From the perspective of inhibiting hydrogen excess adsorption, nitrogen as a cushion gas is superior to methane. From the perspective of selectivity coefficient changes, increasing pressure may lead to a rapid decrease in montmorillonite's selectivity for hydrogen, and the performance of different cushion gases under high-pressure conditions needs further in-depth investigation. The micro-competitive adsorption mechanisms and laws revealed in this study, although based on an idealized model, can provide theoretical guidance for caprock sealing evaluation, cushion gas selection, and optimization of operation strategies in UHS engineering. Future research should focus on constructing more complex models closer to real geological conditions and combining them with indoor experimental verification, to obtain conclusions with greater engineering application value.