The invention discloses an ultrasonic testing method of a steam turbine rotor provided with a center hole. The method adopts an ultrasonic phased array flaw detector and a concave phased array probe; during use, the concave phased array probe is arranged at the certain position of the center hole of a rotor shaft; the concave phased array probe is in well contact with the arc surface of the center hole by a coupling agent; electric scanning is carried out in the same depth along the circumferential direction of the probe; when 360-degree scanning is carried out, the ultrasonic test for the certain section of the rotor shaft can be completed; the concave phased array probe is moved along the axial position of the center hole of the rotor shaft, and the steps are repeated until the concave phased array probe is moved to the deepest part of the center hole of the rotor, and therefore, the ultrasonic test for the whole rotor shaft is completed. After an ultrasonic phased array detection technology is adopted, the method is convenient and rapid to operate, and the steam turbine rotor provided with the center hole can be effectively detected by the method.