Abstract: Based on the transport model of pore gas in unsaturated soil under the membrane, the solution of the transport model of pore gas under membrane is obtained by means of the solution method of heat conduction model. The rationality of the numerical simulation method is verified by using the theoretical formula method of pore pressure calculation under the membrane. The pore pressure in the unsaturated soil under the membrane is simulated and calculated when groundwater levels different rise of heights, and the pore pressure distribution law is analyzed. The influence of exhaust ditch spacing on pore pressure is discussed. The research findings show that in plain reservoirs with geomembrane reservoir basin seepage control, when the infiltrated water is in contact with groundwater, a space of closed gas will be formed, and the rise of groundwater level will cause geomembrane air expansion. The rising groundwater level’s magnitude affects the air pressure distribution in the soil under the geomembrane membrane, and the air pressure under the membrane grows non-linearly with the rising groundwater level. When the rise of the groundwater level causes the increase in the pressure under the membrane, eight kinds of exhaust ditches with different spacing of 30～175 m are set, and the pressure under the membrane is compared and analyzed. It is found that the exhaust ditches can effectively slow down the pressure increase during the groundwater rising process and accelerate the gas migration under the membrane. At the same time, the distance between exhaust ditches affects the exhaust effect. The greater the distance between exhaust ditches, the smaller the drop in air pressure and the more unfavorable the dissipation of air pressure. For typical examples, exhaust ditches with 75 m spacing are recommended.