Abstract: This paper establishes one plane FEM numerical analysis model based on Drucker-Prager elastic-plastic constitutive relation to simulate excavating and supporting of foundation pit. By means of this model combining with actual engineering it analyses the distribution rule of stress field and plastic zone and the deformation mechanism of displacement, sedimentation and bottom heave during excavation and retaining and finds the main factors that affect the deformation, that is, stiffness coefficient and stiffness of fulcrum installing and depth of excavation and soil parameters.