Abstract: Forecasting the position of crack at rear of rock slope is very useful for deformation monitoring and landslide controlling. Based on the theory of energy method, the author establishes a forecasting model used to ascertain crack's position at rear of rock slope. By means of strength reduction method, the function of forecasting the crack's position and safety factor can be deduced. At the basis of a typical case study, we can draw a conclusion that there are some intervals between back crack and slope shoulder; moreover, correctness of the new method has been validated.