The utility model belongs to the technical fields of ultrasonic non-destructive testing of nuclear power station steam turbine vanes and other steam turbine vanes and particularly relates to an ultrasonic automatic checking device for vane roots. Two ends of a flexible guide rail are fixed at two sides ends of a steam turbine vane by a fixing fork and a locking fork respectively; steel wire rope guiding blocks are uniformly distributed between the steam turbine vane and the flexible guide rail; a steel wire rope driving guiding hole is formed in the middle of each steel wire rope guiding block; a steel wire rope penetrates through the steel wire rope driving guiding holes to form a closed loop; the steel wire rope can move along the flexible guide rail; a scanning arm is connected with the steel wire rope and can slide along the flexible guide rail; a motor driver is mounted on the flexible guide rail and is close to the outer side of the locking fork; a positioning rack is fixed on the flexible guide rail and located between the scanning arm and the locking fork; and the steel wire rope is driven by the motor driver to move to drive the scanning arm to move on the flexible guide rail. The ultrasonic automatic checking device for vane roots can be adapted to the situations of small available space for checking and complex contour structures of tested objects.