In order to calculate the ultimate relative displacement of primary support of mountaincrossing highway tunnel, the HoekBrown criterion is adopted; the highway tunnel standards, crosssection pattern, supporting methods and material properties are taken as references; short bench excavation method is adopted; and the ultimate relative displacements of crown top, arch springer and arch feet of the threelane highway tunnel under different grades of surrounding rocks and buried depths are simulated and calculated by software FLAC3D and are further determined by catastrophe theory. According to the statistical analysis of calculation results, it is pointed out that the deformation control points of the tunnel surrounding can be defined clearly and the buried depth can be refined; by analyzing the influences of benches excavation on deformation of different points and comparing to field construction, the influence of the excavation step on the displacement of tunnel surrounding can be determined.