The utility model relates to a reference plate for calibrating a nuclear density meter. The reference plate is composed of a main plate 1, a clamping slot 2 and a fixed plate 3, wherein the clamping slot 2 and the fixed plate 3 are oppositely formed in/arranged on two ends of the main plate 1 respectively; the clamping slot 2 is formed by two trapezoidal plates with the same size and the upper surface of the main plate 1; and the fixed plate 3 is provided with two threaded holes 4 which are distributed along a transverse axis of symmetry of the fixed plate 3. The reference plate disclosed by the utility model has the advantages that the time of calibrating the density meter is short, the time is saved, the working efficiency is improved, the operation is facilitated, and the technical and artificial factors influencing precision are avoided.