Abstract: With the deepening of railway engineering in west Sichuan, a large number of tunnel rock masses in alpine region are affected by freeze-thaw effect. Freeze-thaw cycles lead to the deterioration of surrounding rock properties, which is one of the important factors to reduce rock strength. In this paper, the granite of a railway tunnel along Ganzi-Kangding is taken as the research object. The nanoVoxel-4000 series CT scanner is used to scan the rock samples with high resolution. The threshold segmentation method is used to binarize the scanned images. After stacking the images, a CT scan stereo model is constructed. The box-counting dimension of the sample image after binarization is calculated by fractal theory. Using threshold segmentation method to process the image can better extract the pore distribution characteristics in the image. Combined with the data obtained from the mechanical test, the relationship between the meso-structure evolution and strength change of rock caused by freeze-thaw was explored. The results show that under the action of 20 freeze-thaw cycles, the fractal dimension shows that the internal complexity of rock decreases slightly, but the overall integrity is still good. The internal structure of rock is dominated by unevenly distributed micropores, and the porosity increases gradually. While the complexity of rock structure does not change significantly, the long-term strength and creep characteristics of rock show a decreasing trend. This study provides a reference for the application of fractal theory in rock meso-structure evolution and the study of rock meso-structure and strength evolution.