A detection tool for a big jaw and a small jaw of a CRTS (China Railway Track System) III slab track steel mold comprises a jaw distance measuring plate, two prism tables, contact support legs, a dial gauge, a dial gauge fixing hand wheel and a measuring rod for calibrating readings of the dial gauge; the jaw distance measuring plate is of an inverted U-shaped structure and consists of a flat plate, an inclined plate a and an inclined plate b, the inclined plate a and the inclined plate b are respectively connected with both ends of the flat plate, the two prism tables are mounted on the upper surface of the flat plate in bilateral symmetry manner, and at least two contact support legs are mounted on the lower surface of the flat plate in lateral symmetry manner; a measuring head of the dial gauge and a shaft sleeve outside the measuring head horizontally penetrate inwards through a perforated hole at the lower part of the inclined plate b, the end of the measuring head is closely attached to the surface of the jaw, the dial gauge fixing hand wheel is vertically mounted at the lower part of the inclined plate b, and the dial gauge fixing hand wheel downwards tightens the side surface of the shaft sleeve fixedly. The detection tool changes the deviation of the big jaw and the small jaw of the slab track steel mold into the deviation of the centers of prisms for measurement; and the detection tool for the big jaw and the small jaw of the CRTS III slab track steel mold has the advantages of simple structure, convenient mounting and accurate positioning, and is widely applicable to the detection field of the big jaw and the small jaw of the slab track steel molds.