Abstract: Foundation pit excavation is a foundation engineering and underground engineering construction of an important subject, belongs to a comprehensive geotechnical engineering problems, not only involves the strength and stability of the typical soil mechanics problems, but also contains the deformation and retaining structure and soil interaction issues. With the development of a large number of urban construction, the more and more deep foundation pit engineering, foundation pit digging ever deeper, engineering geological conditions and foundation pit surrounding environment more and more complex, has caused more and more pit accidents. Each kind of supporting technology and design method is based on certain assumptions, such as the formation parameters, the physical model, boundary conditions, etc. But in fact, the geological conditions are complex and changeful, supporting mechanism is not a pattern, there are many uncertain factors in the construction process, with the passage of time,some factors are still changing. Problems of a certain link may cause displacement of foundation pit slope overrun, threat the stability of foundation pit itself and threaten the safe operation of the surrounding buildings, such as the emergency disposal measures are not timely, not when, is likely to cause serious accident. This paper takes Beijing a pile anchor support of foundation pit engineering as an example, the cause of the foundation pit slope deformation induced overrun and control measures are analyzed and summarized.