Abstract: FLAC^3D was used to analyzed a simple slope,the convergence criterion, displacement mutation criterion and plastic zone criterion which were now commonly used to seek safety factor separately. the calculate results of the three criterias were similar,what,s more ,they were close to the tranditional answer of safety factor which was solved by built-in language.It was illustrated that the plastic zone would be more conservative as the criterion of the safety factor， but by monitoring the displacement of key points was relatively straightforward precision. In order to maximize the accuracy, we should take the  other criterias  into account and at the same time based on the above criterias in the future specific using. This would get more reliable results.

Key words: FLAC strength reduction, Safety factor, Failure criterion, Plastic zone, Maximum unbalanced force, Displacement of feature points