Based on the understanding of geological conditions

tunnel excavation response and the physico-mechanical properties from laboratory tests of rock

deformation gradation and safety factor of the soft rock tunnel are proposed. Considering that the root cause of tunnel instability after excavation is the excessive energy of the tunnel native rock; the energy

which is combined with the ratio of the total deformation and elastic deformation

is used to analyze the stability of the tunnel; and on the basis

the safety factor is obtained by increasing-decreasing energy of the original rock. Through the practical application and compared with traditional evaluation methods

the method metioned above is proved to be reasonable and practical. The geological conditions of surrounding rock

the cross section shape

size

excavation and other factors

which are rarely considered by traditional evaluation methods are taken into account

so as to supply important references for the underground engineering.