Abstract: The failure of material can be concluded to the shear fracture and each material has a specific shear sliding surface. The shear strength is the function of normal stress on sliding surface. A series of shear sliding surface is unified and nonlinear unified strength model of geo-materials is proposed. Sliding surface of nonlinear unified strength model is isoclinic surface in β stress space. The Strength surface of nonlinear unified strength model is circular conical surface in the β stress space and a series of conical surfaces are continuous smooth and convex in principal stress space. The nonlinear unified strength model can be illustrated as a curve between the Drucker-Prager and SMP in deviatoric plane, and as a straight line in meridian plane. There are only three mechanical parameters in the model which have definite physical meanings. Compared with large numbers of data under true triaxial tests, the applicability of nonlinear unified strength model is verified to different materials. And the proposed model can describe the nonlinear strength property of various materials reasonably.