The utility model discloses a system for testing the concrete existing compressive pre-stress of a beam bridge cracked cross section tensile region and belongs to the technical field of bridge structural engineering. The system comprises a vertical load applying device, a surface strain gauge and a data acquisition instrument; the vertical load applying device is disposed on a caring cross section of a bridge; the surface acquisition instrument is installed on the concrete edge or outer edge rebar surface at a side of a bridge caring cross section tensile region; the data acquisition instrument is connected onto the surface strain gauge through a conductor; the surface strain gauge is a rebar surface strain gauge; and the vertical load applying device is a jack. Compared with the prior art, the system is simple to operate, accurate and reliable in result, easy for engineering and technical staff to grasp, and suitable for detection and assessment of the pressure eliminating bending moment and effective compressive pre-stress of a pre-stressing concrete bridge cross section.