In plate loading test, the deformation properties of rock mass of surface layer will be different from those of inner layer because of occurring disturbance in the surface layer. But, the difference is usually ignored, and the whole rock mass is treated as homogeneous in existing criterion. This unreasonable mode is improved in this article. In a proposed method, the rock foundation is treated as double layers, i.e., the upper layer is disturbed, and the lower layer is undisturbed. Then a back analytic method based on numerical approach and optimizing theory is developed. The new method has been used to study the deformation parameters of undisturbed rock mass in some project, and the applied results? indicate that the deformation parameter of undisturbed rock mass gotten by the proposed method is more reasonable, and can effectively correct the shortage of conventional analyzing method.