Abstract：With the progress of excavation，the increment of engineering load and the change of environmental factors，soil or rockmass deforms inevitably，and sometimes even instability failure occurs. The purpose of stability monitoring is to predict and find the symptom of instability failure. Generally，the observed external deformation is composed of elastic and plastic deformation of slope rock，shear displacement along the sliding plane and observation error. The shear displacement is important for the evaluation of slope stability，but it is difficult to separate the shear displacement from the measured data. So it is not easy to evaluate and predict stability of rocky slope by external deformation which is small before slope fails. After expounding the external deformation characters of rocky slope and the suspension bridge foundation，the following evaluation principles for the safety of engineering slope are suggested based on the external deformation monitoring：nonsingle-point，conformability，progression，rationality，non-causation and visualization. Then the comprehensive evaluation model is established by means of fuzzy mathematics with multiple factors considered. Based on the boundary conditions and the structure characters of foundation and slope，the zoning safety evaluation on the south bank slope of Jiangyin Yangtze Bridge is given as an example to test this model.
