Ground water is almost always to bring a huge harm to the foundation projects. The change of contact area of earth caused by ground water is analysed, and the formula to calculate the critical angle of planar slide under the saturation condition is derived. The calculated result show that, the smaller the λ (real contact area A of earth mass to slide area A), the smaller the critical angle θ of failure. However, the real interface area A will be lessened with the increase of ground water pressure, which makes the slope resily lose its stability.