Abstract：The mechanical parameters and boundary conditions of landslide often change with time during the process of construction. Traditional quantitative assessing methods of landslide stability，such as limit equilibrium method，finite element method etc.，are pretty difficult to simulate the dynamic mechanical behavior of landslide during the process of construction at the present time，so it cannot achieve the real-time quantitative assessment for landslide stability. A real-time quantitative assessing method of landslide stability in period of constriction is proposed. The new method based on deformation monitoring at depth of landslide may avoid the errors caused by the no determinacy of calculating models，mechanical parameters and boundary conditions etc. According to the respective relationship of the accumulated deformation of sliding surface and slip mass，by calculating the value of Si and Sf，the real-time quantitative assessing the stability of sliding surface from below to above in the progress of construction is conducted. The failure criterion is the peak value of relative deformation. According to the criterion，the symbol of landside instability is the failure of sliding zone which is marked by the destroy of sliding zone¢s hanging wall. When Si = 1 or Sf = 0，the landside is complete stability；when Si = 0 or Sf = 1，the landslide is eventual failure. Different destructive correspond a value between 0 and 1. Moreover，it shows that the method is easy to use and very efficient in application.
