Abstract: Based on the loess tunnel of Lanzhou-Chongqing Railway, mode tests on the instability mechanism of the surrounding rock of large-section loess tunnel are performed. The results show that: (1) The wedge slip is formed on the side-wall parts along the direction of the minimum principal stress firstly and then spread to the vault and arch bottom. The weak tensile strength leads to the loosening collapse on the vault. (2) The instability process is as follows: initiation of local fracture→extension of local fracture→rapid coalescence of fracture→residual strength. The loosening collapse on the vault is on the basis of shear slip on the wall. (3) The radial stress increases as the load applied firstly and then decreases on the vault. The shear stress on the wall increases with the consecutive loading, and the growth rate of tangential stress is greater far away from the hole. (4) The tangential stress-increasing area exists in the deep rock, which bears the self-weight and external soil load and a significant pressure-arch effect is formed. The rock from the wall to the deep part can be divided into three zones: loose zone, pressure-arch zone and initial stress area.