Abstract: The elastic-unloading mechanical calculating model between the existing structure and the new one was established，the solution to additional stress among the whole semi-infinite body was discussed based on the Mindlin solution，and then the analytical formula for additional stress coefficient is derived by using the superposition principles， odd function properties and symmetry of load． The analytic expressions for the unloading vertical stress at any point in the interior of semi-infinite body subject to single and double rectangular uniform load were obtained． The study result can be applied to calculate the unloading-additional stress and the rising deformation of the existing structure induced by construction of the newly-built subway station and the excavations over it．