The invention relates to a fastening device, in particular to an adjustable fastening device for concrete pole cracks. The adjustable fastening device aims at solving the problems that after cracks are generated on an existing concrete pole, the concrete pole is replaced by the adoption of a conventional manner, and circuit operation cannot be guaranteed and lowering the possibility of pole tilting. The adjustable fastening device comprises two clamping element sets and multiple fastening pieces. Each clamping element set comprises two reinforcing rods and multiple hoops, wherein each hoop is a circular-arc-shaped plate, and the two parallel reinforcing rods are fixedly connected to the outer side walls of the corresponding hoops arranged in parallel. Each fastening piece comprises a joint and two bolts, wherein each joint is a hollow cylinder with an internal thread, and the two bolts are screwed to the two ends of the corresponding joint. The two clamping element sets are oppositely arranged. The fastening pieces are arranged on the two hoops in a penetrating manner. The nuts are screwed to the bolts. The two hoops are clamped by screwing up the nuts. The adjustable fastening device is used for fastening the concrete pole cracks.