In this paper, the deformations and the stresses of surrounding rock of Jijiata tunnel, Liulin County, Shanxi Province, are all computed before reinforcement and after reinforcement, through the 3D numerical simulation. According to the computing result, before reinforcement, the computing result of the lateral displacements of the surrounding rock is consistent with the field measurement result. The maximum principle stress and the minimum principle stress are all less than the strength of the surrounding rock supporting, and this will not cause material damage so as to structural damage. But, since the tension stress is higher than the strength of the junction of tunnel arch and tunnel upright wall, this leads the larger deformation and the failure of the junction. After reinforcement, through the steel arch and the lock pin bolt’s enhanced role, the lateral displacement of the tunnel can be constrained effectively, the maximum principle stress and the minimum principle stress can be less than the strength of the surrounding rock supporting, and the tension stress can also be less than the strength of the junction. All the structures are all stable after reinforcement. Thus, this method should be selected as the proper method to reinforce the tunnel.