To investigate the gas permeability performance of unsaturated fine sandy soil, a series of samples with different moisture contents and different dry densities is prepared and air permeability tests are conducted. Based on the porous media permeation theory, the effects of inlet pressure, void ratio, saturation on the air permeability of samples are analyzed, and an empirical formulation of air permeability is proposed. The experimental results demonstrate that air permeability tends to a stable value with the increase of the inlet pressure. The effect of inlet pressure on air permeability is far more obvious for samples with high moisture content than for samples with low moisture content. The increases of moisture content or degree of saturation can result in a decrease in air permeability of sample. When the moisture content is lower than the optimum moisture content, the variation of the air permeability is negligible; however, a rapid decrease in air permeability is observed when the moisture content is higher than the optimum moisture content. The difference of soil microstructure (flocculation structure and dispersion structure) accounts for the air permeability mutation at the two sides of the optimum moisture content. Air void ratio can effectively represent the influence of void ratio and saturation on air permeability. A power function relationship well adapts the relationship of air permeability with air void ratio. Test dates from the literatures also demonstrate the effectiveness of proposed empirical formulation.