Abstract: Large deformation of tunnel surrounding rocks is a sophisticated problem during tunnel construction. Taking the large deformation of surrounding rocks of Milashan tunnel in Tibet for example, the causes of large deformation were studied by site investigation and experimental analysis. The results show that properties of tuff such as poor weather resistance and softening upon meeting water are the two radical reasons of large deformation, and improper construction technology is a main inducement. A treatment scheme and control measures were proposed based on an analysis of causes of large deformation of Milashan tunnel