Abstract: Ground water is the key factor that does more harm to the foundation pit project.How to analyze and examine the effect of ground water on the project is a challenging and important subject.This paper,analyzes the change of interface with the variety of ground water by taking into account the destruction effect of ground water.It derives the equation to calculate the critical angel of planar slide under saturated condition.When the area of interface A* and are smaller,the critical angel of failure is smaller.However,the increase of ground water pressure can reduce the area of interface A*,which shows the ground water pressure makes the soil mass is easier to fail.