Abstract: Earth-cover landfills are one of the primary means of treating urban garbage. However, the organic matter in the garbage generates a large amount of gas upon degradation. The internal gas pressure will be substantially high if the gas generation rate is considerably high or if the gas drainage is not smooth, resulting in the deformation and destruction of the soil cover, thus affecting the stability of the landfill. Accordingly, a plane model test of gas pressure-induced failure of unsaturated soil was performed using a self-designed test device, and the deformation and failure mechanisms of soil under different soil thicknesses and gas pressures were comparably studied through numerical simulation. The results revealed that the process of soil damage induced by gas pressure can be divided into four stages—water and gas migration, local micro crack generation, main crack penetration, and internal cavity formation; Soil damage mainly occurs in the inverted triangle area between the top of the inflatable hole and the surface layer. The soil failure modes can be divided into two types—splitting and burst failure—depending on whether there was a previous gas-pressure effect. The failure pressure of soil increases in an approximately linear fashion with an increase in the thickness of the overlying soil. Accordingly, the concept of failure stress ratio was proposed, and it was observed that the failure stress ratio of each soil layer thickness can be approximately regarded as a constant, which has a certain importance for monitoring the landfill overburden in practical engineering. Additionally, the test results revealed that appropriate gas pressure is conducive to increasing the stability of soil mass; the soil mass will gradually become unstable if gas pressure exceeds a certain value, based on which the concept of critical stable gas pressure was proposed. Furthermore, the proposed numerical simulation method was used to establish a corresponding two-dimensional numerical model with reference to the model test. The numerical results, including the fracture propagation pattern and failure pressure results, were consistent with the model test results. On this basis, the seepage variation law within the soil mass was deeply studied. It was observed that the gas pressure increases the pore pressure inside the soil while driving the water to diffuse around, causing changes in the saturation of the surrounding soil. Finally, the simulation results revealed that regional change of effective stress increment may be the cause of critical stable gas pressure, providing a reference for practical engineering.