The utility model relates to an adjustable signboard mounting rack, which includes a rod number board or a warning board, two parallel rods, two crossed rods and bolts, wherein an elongated hole is formed in the upper part of each of the parallel rods, and the two elongated holes are mutually symmetrical; the two crossed rods are crossed and hinged at the middle; the lower end points of the two crossed rods are hinged to the lower parts of the two parallel rods; the two upper end points of the two crossed rods are fixedly connected with the elongated holes in the upper parts of the two parallel rods through the bolts; the rod number board or the warning board is fixedly mounted on the parallel rods. The mounting rack has a simple structure, is convenient and quick to use, saves manpower and reduces labor intensity and material waste, prolongs the service life of the signboard, has the advantages of neat and attractive appearance, firmness, reliability and the like, and can be widely applied in mounting of rod number boards or warning boards on steel angle towers and communication towers of an electric transmission line.