Abstract: The French drains beneath the geomembrane are an important engineering measurement for the geomembrane as anti-seepage structures of reservoirs, and have a decisive effect on the air exhaust beneath the geomembrane. The conversion relationship between the permeability coefficient and the square of path length is proposed on the basis of the principle of equivalent flow variation. It is deduced that the important reasons for the bulge of the geomembrane are the huge reservoir area and the long drain path because of the air resistance of French drains. The simplified method for consolidation of unsaturated soils is used to calculate and simulate the air pressure distribution beneath the geomembrane under the condition of vacuuming. The comparison between the measured and calculated values in the field tests has verified the rationality of the proposed method and parameters. The results reveal that a pressure changing zone and a stable zone exist along the horizontal direction whether there is a French drain or not. The pore-air pressure changes significantly in the zone of a range of 2.5 m along the horizontal direction from the vacuuming spot, which indicates the existence of the air resistance of French drain. The distribution of pressure gradient beneath the geomembrane reflects the change of the air resistance of French drain. The main influencing factors for the air resistance of French drain are the air intake value and the degree of saturation of materials. Reducing the air intake value and lowering the degree of saturation are beneficial to reducing the air resistance of French drain. The research results may provide a scientific theoretical basis for the evaluation of the design of anti-seepage scheme of the geomembrane.