The invention discloses an automatic ultrasonic fault detector, aiming at solving the problems that a coupling agent cannot be sprayed automatically, test pieces cannot rotate automatically, the location precision of a probe is low, and three-dimensional and automatic fault detection is hard to realize. The automatic ultrasonic fault detector comprises a guide rail part, a clamping tool part and a probe part, wherein the guide rail part comprises a guide rail housing (1) and a sliding block, the clamping tool part comprises a driving clamping head (7) and a driven clamping head (9), the probe part comprises a probe (17) and a probe lifter (20), the probe part is mounted on and in bolt connection with the top end face of the sliding block in the guide rail part, the upper end of the probe part extends out from a rectangular through hole in a cover of the guide rail housing (1) in the guide rail part, the driving clamping head (7) and the driven clamping head (9) in the clamping tool part are arranged at two ends of the guide rail housing (1) in the guide rail part, and the horizontal top end face of the probe (17) in the probe part is coplanar with the connecting line of rotation centers of the driving clamping head (7) and the driven clamping head (9).