Abstract:For a long time,stability analysis methods of surrounding rocks in tunnel are unscientific and unreasonable. It still judges the stability of tunnel by the empirical criterion of displacements of tunnel perimeter or sizes of plastic zones of surrounding rocks. Displacements of tunnel perimeter are affected by elastic modulus of surrounding rocks or shape and size of tunnel,and displacements of tunnel perimeter are different at different positions. So it is difficult to get a unified displacement criterion standard. Judging the stability of surrounding of rocks by sizes of plastic zones is superior to displacement criterion. But plastic zones are affected by Poisson¢s ratio or shape and size of tunnel and size of plastic zones calculated by different softwares are different. The method is also unreliable. So the traditional empirical methods are unreasonable. The paper puts forward safety factors based on the strength reduction finite element method as the stability criterion. This criterion is based on the strict mechanical foundation,which has a universal standard and can not be affected by other factors. Taking a loess tunnel for example,the concepts of shear safety factor and tensile safety factor of a tunnel are proposed. With the reduction of c,j or c,j and tensile,plastic zones of rock mass keeping expanding,when the value of the nodal displacement or plastic strain has a big jump compared with that before failure,this means that loess tunnel reaches shear failure,and the reduction factor is just the shear safety factor. With the tensile reduction,loess tunnel reaches tensile failure when the first element of rock mass around loess tunnel free face fails in tension except for the bottom free face,and the reduction factor is just the tensile safety factor.