Ability insufficiently in shear is one of common trouble of old RC (reinforced concrete) beams, so scientific and reasonable shear resistant reinforcement method is one of the important research topics in the old bridge reinforcement and rehabilitation fields, and is of great significance in improving old bridge’s mechanical properties and reducing safe accidents. The shear resistant strengthening effect of strengthened beams with SWR (steel wire rope) external prestressing is studied under different dead load level and damage degree when strengthening, through the indoor experiment in shear. The results of the experiment shows: the width of diagonal cracks of the beams strengthened with SWR external prestressing become smaller and the yield load of stirrups is delayed, and the shear capacity is improved compared with basic beam. Both dead load level when strengthening and damage degree of original beams are the important factors influencing the shear resistant reinforcement effect , and with increasing of the dead load level and damage degree when strengthening, the improved amplitude of the yield load of stirrups and the shear bearing capacity are getting decreasing. The dead load level and damage degree when strengthening should be taken into account.