Abstract: The nonlinear unified strength theory in β stress space is similar to the Drucker-Prager strength theory in principal stress space. Compared with the establishment of Cam-clay model with stress parameters of p and q, a new three-dimensional elastoplastic constitutive model is established in β stress space. A new dilatancy equation is proposed, and then plastic potential function is obtained combined with the orthogonality condition. The new model is based on the nonassociated flow rule and yield function of modified Cam-clay model. It is a new idea to establish three-dimensional elastoplastic constitutive model directly. The proposed model in which the failure condition is based on nonlinear unified strength theory can reflect the deformation and strength characteristics of soils reasonably, and can be devolved into the Cam-clay model.