The invention provides a device for detecting large and small jaws of a steel die of a CRTS III type track slab. The device comprises a jaw ranging plate, prism seats, contact legs, a dial indicator, a dial indicator fixing hand wheel and a measuring bar for demarcating the reading of the dial indicator, wherein the jaw ranging plate has an inverse U-shaped structure consisting of a flat plate and inclined plates which are connected to two ends of the flat plate respectively; the prism seats are symmetrically arranged on the left and the right of the upper surface of the flat plate; at least two contact legs are symmetrically arranged on the left and the right of the lower surface of the flat plate; and a measuring head of the dial indicator and a shaft sleeve outside the measuring head horizontally pass through through holes on the lower parts of the inclined plates inward, the end part of the measuring head is closely attached to the jaw surface, while the dial indicator fixing handwheel is vertically arrange on the other lower part of the inclined plate, and is pressed against the lateral side of the shaft sleeve downwards. The device can conveniently convert deviation of the large and small jaws of the steel die of the track slab into deviation of the prism center for measurement, has a simple structure and can accurately position the jaws, is convenient to assemble, and can be widely used in the field of detecting the large and small jaws of the steel die of the track slab.