The combination of a single row of artificially-dug-up piles and pre-stressed anchor rope was adopted to the foundation pit as the retaining structure,and anchor-plate retaining structure and rubble retaining wall were used as the bracing system for the complicated building slope. In the process of excavation, monitoring has been done to see if any horizontal displacement, deformation of deep soil and crack width take place or any technical data and code dispatch from the demand. Monitoring results show that deformation in some parts of the tops of support structure is larger than the designed warning value some times and the deformation continue to increase in the process of the excavation, and this result in twice dangerous situations in the support structure of the slope. In this paper, the geologic construction of the slope and the situations of the designed support structure and monitoring condition were depicted. The mechanism of the instability of the slope was explained on the basis of comprehensive analysis of deformation monitoring datas. It is concluded that the over-excavation, explosive excavation, rainfall and so on were the factors affecting the stability of the slope. Corresponding emergency measures and reinforcement measures,such as unloading and pre-stress rope, were proposed, which were proved to be highly effective.