Abstract：By the computerized tomography(CT) technology，the properties of damage propagation of sandy mudstone，on top and waist of Fenghuo Shan tunnel in Tibetan plateau，are studied under the freeze-thaw cycle condition. The results show that water migration and volumetric expansion 9% are the main factors resulting in the freeze-thaw damage of sandy mudstone，and the extent and range of damages are varied with the amount of water. In other words，the damage propagation is rather fast in the first 6 cycles，and then almost keeps constant with a little water-supply or none. But under the condition of large water supply，the damage is always developing. Whatever condition the rock sample is in，the side region is damaged more easily and severely than the others. Moreover，during freeze-thaw disturbance of loading and unloading，moderate weathering of sandy mudstone providing passage for the water migration and vast water supply increase the speed of damage. This indicates that the stability of soft-rock regions with many joints or cracks is not favorable，when the rock is penetrated by enough water in freeze-thaw process.
