Abstract: The strength criterion in the characteristic stress space, whose form is the same as that of the Drucker-Prager strength criterion, can describe the true three-dimensional strength of geomaterials. In other words, the isotropic function in the characteristic stress space can describe the anisotropic mechanical properties of the geomaterials. In the characteristic stress space, a new yield function is proposed for the normally consolidated soils on the basis of the interpolation function method. By using the plastic potential function from the authors’ previous researches, a true 3D elastoplastic constitutive model for normally consolidated soils is directly established in the characteristic stress space. There are only 7 parameters in the proposed model. All the parameters have clear physical meanings and can be easily determined through laboratory tests. The proposed model is analyzed through model simulations and is verified using the soil data available in the literatures. These results demonstrate that the proposed model can simply and reasonably describe the characteristics of the true three-dimensional strength and deformation of normally consolidated soils and can be degenerated to the modified Cam-clay model.