Abstract: Probing method is established using critical sliding field theory on basis of surplus force method.Sliding surface and optimum slope angle in given safe coefficient is decided.Author relies on deformation behavior of combined open pit slop and hill.Reserve probe is carried using expending method of critical sliding field.Safe coefficient is given.Maximum angle of slope is probed and optimum design is carried with history-case.So optimum slope angle is designed and original design.is corrected,therefore,mine safety and resource retrieve coefficient is assured.