A new method to define slippage line under local loading condition is presented based on Hill’s principle of strain bifurcation and the elastoplastic constitutive model of soil. It can tail how slippage line formed and developed

and can reveal process of onset and evolution of slippage point and directions in elastoplastic slope body also. In practice

it is convenience to define the location and movement direction of slippage line in the slope by the elastoplastic continuum with finite element increment numerical method. During each step in elastoplasticity

whether the determinant of acoustic tensor on each Gauss point equals zero or not should be checked

once it equals zero or near to zero

mark the point and the movement direction. With some points are marked and arrayed regularly

the slippage line will be formed.