In order to make rodless drilling rigs support the hole-wall soil effectively,the stability of hole-wall soil affected by the prestress exerted on support structure is analyzed,and a method,which uses virtual soil column to simulate pile hole and combines with the Mindlin solution,is proposed.With the help of this method,the additional stress vector of any point in the prestressed hole-wall soil is deduced,on the basis of which the shear stress and its relative strength of any point in the location of 50mm in width and 5mm below the support plate are obtained under the actions of maximum pre-torque and maximum pre-pressure.The results show that,when the pre-torque and the prestress are respectively 120kN·m and 100kN,the maximum shear stress reaches 6. 52MPa; and that the relative strength of shear stress at each point is less than 1 except at both ends of the support plate,which means that the hole-wall soil is in a steady state overall.Moreover,a comparison is made between the calculated results and the experimental ones,finding that the proposed method is more accurate than the Boussinesq-Gerruti solution and the finite element method.