Coating can protect the exterior wood products from aging caused by environmental factors. This paper researches the properties of coating and wood during aging by measuring the air permeability of exterior coated wood. The results show that the anti-aging ability of coating has no clear relation to the permeability of coating. The air permeability of exterior coated wood is lower than that of the uncoated wood, and it increases with the aging time. The coating on the radial wood surface is stronger than that on the tangential surface, and the permeability uptake coefficient kt on tangential wood surface has more cleat relation with aging time: the permeability uptake coefficient kt increase quickly at first aging period, then increase slowly at middle aging period. At last aging period kt even decrease.