Abstract: A method for back analysis of the initial geostress is discussed. The fitting accuracy is improved remarkably by regressing each stress component respectively and using the partial least squares regression method. By loading the regression results of in-situ stress component to model elements, the relationship between the element stress and node unbalanced force is analyzed on the basis of the principle of FLAC^3D. An external force equal to the reverse force of the node unbalance force can be calculated, which can be loaded on the nodes to satisfy the equilibrium conditions. For the elements that dissatisfy the strength conditions, the stress is modified to satisfy the strength conditions by assuming the average stress invariant and reducing the partial stress. This step has also solved the unreasonable distribution of the boundary nodal forces. The modified stress field satisfies the equilibrium conditions and regression value as well. A three-dimensional hypersurface spline interpolation method is developed to calculate the stress tensor at any point. Finally,an engineering example shows that the proposed method improves the fitting precision of local stress at abnormal area.