In Northwest China, loess is widely distributed and is the main material for landfill final cover. The gas permeability of loess will effect the landfill gas emission directly. In order to study the effect of the water content on the gas permeability characteristics of the loess cover in service, a device is developed to measure the gas permeability of unsaturated compacted loess specimens. The water content of the specimens after compaction is controlled using the osmotic technique. In addition, a one-dimensional steady-state model of gas migration in waste layers and final cover is established on the basis of the flow theory in porous media. The effects of gas permeability of the loess cover and gas extraction rate in the diffusion layer on landfill gas emission are investigated. The experiments demonstrate that the osmotic technique can effectively simulate the changing service water content of the loess cover. The measured gas permeability of loess specimens ranges from 10-17 m2 to 10-12 m2, and decreases with the increase of service water content; and the decrease is more significant for the high density specimens. The gas pressure at the bottom of the loess cover increases with the decrease of the gas permeability. Extracting landfill gas from the gas diffusion layer of the cover is able to reduce the gas pressure and the landfill gas emission rate.