As the implementations of the western development in China, more and more tunnels will get through the western mountains in China. In order to economize the construction costs, a new type of underground structural form called branching-out tunnel must be applied. The failure process of the branching-out tunnel under lateral overload action is also greatly complicated, which is related with the depth of the mountains, the branching-out angle, the in-situ stress field, the thickness of the middle wall and so on. This paper uses the 3D-physical model of geo-mechanical model tests to study the stability and failure process of this complicated structure, especially the part of the middle wall. The physical model is built up in a new kind of analogy material. In the process of the whole experiments, different lateral pressures imposed on both of the lateral planes of the physical model. According to different lateral pressures, we have attained the change of the stress and displacement field and looked into the failure process of the pivotal positions in the branching-out tunnel. We also use finite element method analysis software RFPA (Realistic Failure Process Analysis) to simulate the whole failure process of the branching-out tunnel. Finally, we have got the load-bearing safety reliability of this complicated structure through comparative analysis of physical modeling and numerical simulation.