A detection tool for the length, width and thickness of a CRTS (China Railway Track System) III slab track steel mold comprises a detection bottom plate, wherein a prism sleeve for mounting a prism is fixed on the upper surface of the detection bottom plate, a prism mounting hole is vertically machined at the center of the prism sleeve, a connecting rod at the bottom of the prism is fixed in the prism mounting hole, a tightening mounting hole communicated with the prism mounting hole is machined on the side wall of the prism sleeve, a hand wheel mandril is in threaded connection with the tightening mounting hole, and the head of the hand wheel mandril butts against and tightens the side surface of the connecting rod; positioning cones closely attached to two side edges of the inner angle of the steel mold are arranged on the lower surface of the detection bottom plate, and the angle point of the inner angle of the steel mold corresponds to the center of the prism up and down; and a row of magnet steels are arranged on the detection bottom plate and correspond to the edge of the steel mold. The slab track steel mold detected by the detection tool meets the accuracy requirements on length, width and thickness deviation of the steel mold specified in the specifications, so as to improve the detection efficiency for the length, width and thickness of the slab track steel mold. The detection tool for the length, width and thickness of the CRTS III slab track steel mold can be widely applicable to the detection field of slab track steel molds for passenger special lines.