The coatings can protect the exterior wood from being invaded by air and water. Wood is a kind of material with a lot of porous so it can be permeated by gas and liquid. When the coatings are used in wood or porous structures, it should give complete blockage of liquid permeation while at the same time should have ability to release out water vapors. This paper researches the influence of four kinds of exterior coatings on the gas permeability ability of beech in the direction of radial and tangential. The experimental data prove that the film can protect well the wood from being invaded by air and water, and these four kinds of coatings all reduce the gas permeability of beech after coating. The solvent-borne coating is more airtight than the water-borne and the colour coating is harder to be permeated than the varnish. The gas permeability on beech substrate in radial direction is a little stronger than that in tangential direction in any case of before or after coating.