The application discloses an automatic span adjusting device for an automatic bending test. The automatic adjusting device comprises a support frame, a sliding rail, sliding blocks, a servo motor, a synchronous belt, a bidirectional ball screw, supporting bodies, lead screw nuts, a lead screw fixed end and a motor bracket; the support frame main body is a device support main body; sliding blocks are symmetrically arranged at two ends of the sliding rail; supporting bodies are the sliding blocks; lead screw nuts are fixed on the supporting bodies; a servo motor is fixed on the motor bracket; the servo motor drives the bidirectional ball screw through the synchronous belt; the bidirectional ball screw is matched with the lead screw nuts, and when the bidirectional ball screw rotates towardsone direction, the lead screw nuts move towards the inner side or the outer side at equal intervals, thereby driving the supporting bodies to move. The span can be automatically adjusted from 32mm to160mm under the drive of the control system, and the adjustment precision can reach 0.01 mm. The span adjustment precision and the test efficiency are improved, the labor intensity of an operator is reduced, and human errors are avoided.