In addition to vertical pulling force, the ground-drive screw pump oil recovery rod also stands twisting moment and bending moment. The diameter of the polish rod is generally larger than that of the pumping rod, and it stands a bit less stress than the pumping rod close to it, and the polish rod is generally made of relatively better materials. In practice, however, the polish rod fractures more often than the pumping rod above. This thesis considers that if the driving head deflects, the upper and the lower polish rods of the packing set are not in the same axial line, and bending moment will come into being in the polish rod of the lower part of the packing set due to the restriction of the polish rod by the packing set. Alternating stress caused by bending moment yields fatigue rupture of the polish rod during its rotation. This verdict is confirmed by the fracture position, the shape of the fractured surface, and quantitative calculation. This thesis proposes strict controlling of the deflection of the driving head by means of polish rod verticality survey instrument. Both theory and practice have proved that this method can effectively avoid fatigue rupture of the polish rod.