The utility model belongs to the technical field of bridge structure engineering, especially to a system for reducing the measuring error of the existing stress of a steel bar of a prestressed concrete bridge. The system comprises a steel bar, a flame cutting machine, a high-temperature strain gauge, a cooling apparatus, a concrete protective layer chiseling-off area, a data acquisition instrument and a stress releasing point, wherein the steel bar is wrapped in the concrete protective layer chiseling-off area, the high-temperature strain gauge is pasted to one end of the steel bar along the axis direction of the steel bar, the stress release point is at the other end of the steel bar, the cooling apparatus is arranged between the high-temperature strain gauge and the stress release point and situated at 5cm away from the stress release point, the data acquisition instrument is connected with the high-temperature strain gauge, and the flame cutting machine is used for cutting the steel bar at the stress release point. Compared with the prior art, the system is simple in operation, high in accuracy and reliable in result, and is applicable to selective examination, detection and evaluation of existing stress states of steel bars at prestressed concrete control interfaces.