A new method to express the strength condition of the anisotropic soil is proposed. Based on the method of matrix transformation used in the compound material mechanics, the anisotropic flexibility matrix (or stiffness matrix) of the anisotropic sand with a certain stress state is transformed to be an isotopic one of a certain isotropic reference material with the corresponding stress state, and a corresponding stress matrix is obtained simultaneously. By this step, the anisotropic soil material is transformed to the isotropic reference material. Then, the isotropic strength theory is applied on this isotropic reference material. Finally, the isotropy strength theory is expressed by the original stress matrix of the anisotropic soil. The calculation results are quite matched with the experiment results.