Abstract: The initial stress state and stability of the surrounding rock mass will change during tunnel excavation. The rock mass even may collapse due to over-size displacement. A new method is presented in this paper for reliability analysis of tunnel excavation. The genetic algorithm is put forward to calculate the reliability index and failure probability for the complexity of the model. The proposed method is applied to reliability analyzing of a practical expressway tunnel. The result shows that the potential break areas appear at vault and spandrel of the tunnel. The reliability indexes and failure probabilities of the vault settlement and spandrel convergent reflect that the reliability of the unsupported construction can't meet the requirement of the specification. The tunnel must be supported in time.