Abstract: Based on the Yuelongmen Tunnel on the Chengdu-Lanzhou Railway, a combined method of field moni? toring and finite element calculation is used to explore the causes for the primary support failure and the action mechanism of secondary support in the parallel adit of tunnels in extremely-squeezed surrounding rocks. Firstly, according to the deformation monitoring data of the adit, the deformation characteristics of the adit and the restraining effects of different measures on deformation are analyzed. Then, on the basis of the Rock Mass Rating (RMR) method,reasonable physical and mechanical parameters of the surrounding rock are selected, and the deformation and the stress of the adit support are calculated using elastic-plastic finite element analysis, and the action mechanism of the double-layer support is discussed. The study shows that measures such as adding bolts and grouting reinforcement can slow down the deformation rate of the tunnel, but the control effect on the accumulated deformation of the tunnel is not significant. The application of the secondary support can change the stress state of the primary support,increase the bearing capacity of the primary support by about 12%~14%, and can effectively control the deformation of surrounding rock.