Abstract: The interpolation of the plastic modulus between the real and imaginary stresses is used to determine the actual nonlinear deformation of the clay. The bounding surface model is suitable for the simulation of small nonlinear deformation of the clay. An anisotropic bounding surface model is developed based on Wheeler's elastoplastic model (S-CLAY1) and the bounding surface theory. The anisotropy of the natural clay can be captured by the model. Several problems on the theory of bounding surface model are discussed. The paper provides some guidance to understand and master the theory of the bounding surface model and to develop a new bounding surface model.