Strength reduction finite element method is used in road tunnel. Based on latent slip surface and safety factor

the rationality of the designed parameters of support and excavations can be judged. The result shows that the distribution of plastic zone is effected greatly by Poisson’s ratios; however

safety factor varies little with it. The higher quality of rock

the more distribution of plastic zone under limit state is got. But the failure zone is lower. Additionally

the more thickness of upper rock layer

the more distribution of plastic zone under limit state is found. Meanwhile

the safety factor decreases. The example also shows that the plastic zone of good rock quality is larger than that of poor rock quality at failure condition. So it is deserved to study about judging safety of tunnel only by its plastic zone.