According to the entire theory of plastic mechanics, the relationship between equivalent stress and equivalent strain of a surrounding rock is deduced from a nonlinear softening constitutive model of uniaxial compression. The surrounding rock is classified into loosened zone, plastic zone and elastic zone, and an elastoplastic analysis is performed for a lined pressure tunnel under the equivalent load. The displacement and stress distribution laws at the loosened zone, plastic zone and lined zone are presented. On the basis of this, the formula of rock resisting coefficient is given. The formula not only considers the intermediate principal stress effect, but also is similar to other researchers formulas.