Context <p>The efficient development of deep coalbed CH₄ resources is strongly constrained by coal moisture, and this effect is particularly pronounced in high-moisture lignite. However, the molecular mechanism by which moisture influences CH₄ storage and migration in lignite remains insufficiently understood. To clarify this issue, this study investigates the effects of moisture on the occurrence characteristics of CH₄ in Shanxi lignite, with a particular focus on pore occupation, adsorption behavior, and diffusion performance under different moisture conditions.</p> Methods <p>Monte Carlo and molecular dynamics (MD) simulations were employed to construct pore structure models of Shanxi lignite at various moisture contents, and CH₄ adsorption isotherms, adsorption energy distributions, radial distribution functions (RDFs), and mean square displacements (MSDs) were systematically analyzed. The results show that, with increasing moisture content, pores larger than 5&#xa0;Å are progressively occupied by water, substantially reducing the accessible space available for CH₄. Under different temperature-pressure-moisture conditions, moisture markedly weakens the CH₄ adsorption capacity of lignite, and this inhibitory effect becomes more pronounced at elevated temperatures. When the moisture content reaches 4 wt%, CH₄ adsorption decreases by approximately 30–40%. Water preferentially occupies high-energy adsorption sites, forcing CH₄ to migrate toward energetically weaker regions and significantly suppressing its diffusivity, with a reduction exceeding 40%. Pore size and connectivity are identified as the key factors controlling CH₄ adsorption and migration. These findings provide molecular-level insights into CH₄ occurrence in high-moisture low-rank coal and offer guidance for optimizing coalbed methane development strategies.</p>