The competitive adsorption behavior of H₂/CH₄ mixtures in depleted shale gas reservoirs is a key factor in determining the potential of underground hydrogen storage (UHS). This paper, based on an established Type IIC model of the kerogen matrix, using grand canonical Monte Carlo (GCMC) simulations method to measure the pure and binary adsorption of CH₄/H₂ on shale kerogen. The results indicated that the adsorption capacity of pure CH₄ is 2–4 times that of H₂ under the simulated condition, which is attributed to the lower adsorption heat and weaker Van der Waals interactions of H₂ compared to CH₄. In a binary CH₄/H₂ environment, CH₄ is preferentially adsorbed on kerogen, and the total adsorption of CH₄/H₂ is positively related to the molar fraction of CH₄. The adsorption selectivity of CH4 over H2 (S_CH4/H2) is greater than 1, which is influenced by pressure, temperature, and CH₄/H₂ molar fraction. SCH₄/H₂ initially declines and then leveled out as pressure increased, which helps ensure a high purity of the produced H2. This study demonstrates that shale kerogen can adsorb 0.66–0.79 mmol/g of H2, corresponding to 2.54–4.23 × 104 m³ per well. Under the studied conditions, operating above 20 MPa facilitates the extraction of high-purity H2. The research provides theoretical guidelines for estimating H₂ storage capacity and optimizing operation pressures for UHS.