Abstract: In view of the support replacement problem induced by support failure (exclusive of invert arch) in the soft rock large deformation tunnel on Jiuzhaigou-Mianyang Expressway, this study explores the mechanical behavior and spatio-temporal deformation characteristics during support replacement through field monitoring and numerical simulation methods. The results show that: (1) Under the research conditions, the new support near the non-replacement section bears less load, while the new support further away the non-replacement section bears more load.Among the new supports at the same distance from the non-replacement section, the one replaced earlier bears more load, and the one replaced later bears less load. (2) The position of the support replacement working face has a greater impact on the load of the new initial support than on the secondary lining, and the proportion of stress sharing between the new initial support and the secondary lining is positively correlated with the relaxation stress caused by the replacement. Conducting support replacement after improving the physical and mechanical parameters of surrounding rocks is an effective measure to solve the large deformation problem in soft rocks. (3) The displacement of the structure within 1D (D is the equivalent diameter of the tunnel) in front of and behind the non-replacement section is less affected by the support replacement, and the displacement in the middle part of the replacement section is basically the same and larger than that at both ends. (4) For tunnels with poor rock types such as soft rocks, necessary temporary protection should be applied within a range of at least 1D in front of and behind the replacement position. Grouting before replacement can play a role in pre-reinforcement, and the process of "failure-grouting-replacement-resistance" can effectively suppress the rheological effect of soft rocks.