The utility model relates to a detection tooling of finished product plate size jaws of cathode-ray tubes (CRTS) III type rail plates. A detection bottom plate is in a rectangular shape, the center of the detection bottom plate is provided with a prism installing hole and is provided with a prism, a connecting rod arranged at the bottom of a prism frame is fixed in the prism installing hole arranged in the center of the detection bottom plate, at least three fulcrum screw bolts with the same height are connected onto the detection bottom plate, the bottoms of the fulcrum screw bolts are in contact with rail bottom slopes of finished product plates, the left side edge and the right side edge of the detection bottom plate are respectively provided with two contacts, and the heads of the contacts forwards extend out of the side surface of the detection bottom plate and are in contact with inner inclined surfaces of finished product plate baffle shoulders. The device converts the deviation of the finished product plate size jaws of the rail plates into the deviation of the prism center arranged on the tooling, so the deviation of the finished product plate size jaw parts of the rail plates can be precisely detected through an electronic total station, the precision is ensured on one hand, the efficiency is improved on the other hand, and the detection tooling can be widely used for detecting the finished product plate size jaws of the rail plates in passenger transportation lines.