The invention discloses a stretching mechanism for steel plate stretching detection. The stretching mechanism comprises a rack and two chunk devices; each chunk device comprises a chunk shell, and the chunk shell comprises two shell side wall portions, wherein the middles of the bottom faces of upper top plates are provided with connecting portions, clamping oil cylinders are fixed on the connecting portions and located in the shell side wall portions, push rods of the clamping oil cylinders stretch out of the connecting portions and are fixedly provided with clamping blocks, the upper portions of the clamping blocks are provided with insertion connecting portions which are inserted into insertion grooves formed in the bottom faces of the upper top plates, the bottom faces of the clamping blocks are fixedly provided with sliding moving blocks, and the two ends of at least two guide rods are fixed on the inner side walls of the two shell side wall portions; the outer side walls of the shell side wall portions of the middle chunk device are provided with connecting portions, the connecting portions are provided with threaded connection through holes and limit insertion holes, at least two guide columns are fixed on a top plate of the rack, and at least two rotation double-screw bolts are hinged to the top plate of the rack. According to the stretching mechanism for the steel plate stretching detection, the chunk devices clamp firmly, a driving motor drives two rotation screws to synchronously rotate, and therefore the synchronicity of the mechanism is guaranteed.