Vapor-phase perfluoroalkyl and polyfluoroalkyl substances (PFASs), along with methane emissions from landfills has been key contributors of their atmospheric transport and global distribution. Given the persistence, bioaccumulation, and potential health risks associated with PFAS, understanding their transport behavior in landfill gas barrier is of paramount importance. To gain a deeper understanding of the adsorption and diffusion behavior of vapor-phase PFAS in unsaturated, montmorillonite-rich clay barriers, a molecular dynamics simulation was conducted. A 5-nm montmorillonite nanopore incorporating vapor-phase PFAS (Fluorotelomer alcohol, FTOH), methane, and water molecules was modeled considering the interactions between these species. The results indicate that the presence of methane within the montmorillonite system inhibits the diffusion of both water and FTOH. Additionally, methane competes with FTOH for sorption sites, particularly at low moisture content. At 5 % moisture content, the adsorption density peak of methane is 1.5 times greater than that of FTOH due to stronger van der Waals interactions between methane and montmorillonite. However, as moisture content increases, methane adsorption weakens and becomes more dispersed within the montmorillonite pores. In contrast, FTOH retains a distinct adsorption region at 20 % moisture content, exhibiting a density peak of 0.025 g/cm