Abstract：A constitutive model，which can almost describe the characteristics of rock mass truly，is obtained by perturbation method. The curve of the constitutive model is linked glossily by elastic，strain nonlinear hardening and nonlinear softening segments. The stress distribution laws in plastic hardening and softening zones of surrounding rock under hydrostatic pressure are achieved. Based on the analyses，equilibrium equation of surrounding rock，relationship between geostress and tunnel displacement and characteristic equation of surrounding rock are presented；and the expressions of geostress shared by elastic zone，hardening zone and softening zone of surrounding rocks are given. Calculation shows that the bearing capacity of the hardening zone of surrounding rock is more large than that of elastic zone. Especially，the curve of sq /po-r/a has no up-pinnacled stress concentration；and the result accords well with the experiment one of other scholar. It is pointed out that there are obvious limitations in Kastener formula which is based on ideal elastoplastic constitutive model；and the up-pinnacled stress concentration of sq -r curve does not correspond with the practical one.
