Abstract: Based on a deep foundation pit project in Anning, District of Lanzhou in China, the reasons for the deformation of its supporting slope under excavation process are comprehensively analysed. The further slope deformation is prevented by the temporary treatment measures promptly. Through calculationg, the reasonable supporting schemes of the foundation pit are given after the deformation is controled. A finite element model is established by means of the finite element software. The deformation of the foundation pit under no prestress and prestress conditions on anchors is comparatively analyzed. The calculation shows that the deformation of the foundation pit is significantly controled by the pre-stress. The reliable basis for reasonable support scheme is provided through the calculation and analysis results. Through comparative analysis, the reasons for the deformation are obtained. In order to avoid similar engineering accidents, some suggestions for the design, construction and management of foundation pits are proposed.