Abstract:The criteria of critical state need to be specified in determining the safety factors of rock and soil slopes when the elastoplastic shear strength reduction method is used,but there are some subjective factors in this kind of method and the precision of computation results always depends on the experience of researchers. In this paper,the critical state of rock slopes is studied based on FLAC3D numerical technique and a new method is presented. The convergence of curves of the maximum nodal displacement with time steps is closely related with the stability of slopes. This kind of curve can be used as criterion to determine the stability of slopes. The safe state of slopes can be estimated by whether the maximum nodal displacement-increment decreases during certain time-steps after a specified time-step. This method overcomes the randomness of some criteria in which the permissible error is artificially specified and the computation precision can be controlled according to the need of projects in order to obtain more objective and exact safety factors of slopes. Finally,an example of highway rock slope is studied by this method and more satisfied results are obtained compared with those by Bishop¢s method.