The utility model relates to a ruler for jaws of a mold, which relates to a device for measuring a distance between two jaws of the mode with a retaining shoulder rail bearing platform, and comprises a main ruler, wherein one end of the main ruler is a fixed end; the main ruler is provided with a telescopic end which is fixed on the main ruler through a bolt; and two movable supporting plates are arranged between the fixed end of the main ruler and the telescopic end, and are connected with the main ruler through bolts. Before measurement, corresponding positions of the mold are supported through the two movable supporting plates, so that the main ruler is stabilized without inclination; then the fixed end is clamped on one side of the jaws of the mold to be measured, and the bolts on the movable supporting plates are screwed tightly so as to be fixed; and finally, the telescopic end is moved to be contacted with the other side of the jaws of the mold to be measured, the bolts thereon are screwed tightly, and the reading number of the ruler for the jaws of the mold is the distance between the two jaws of the mode with the retaining shoulder rail bearing platform. The ruler for the jaws of the mold is more convenient and quick to measure, the time is saved, the efficiency is improved, and the labor is saved.