The invention discloses an ultrasonic phase array detection method for an in-service steam turbine rotor blade. The ultrasonic phase array detection method is characterized by comprising the following steps of: 1, selecting a phase array device with a fan-shaped scanning function; 2, detecting the sensitivity of the phase array device; 3, 1) placing a second probe at a perpendicular leaf-shaped outer arc position, moving the second probe in parallel to scan defects of an inner arc, and scanning a longitudinal defect in a manner of being parallel to the leaf shape; and 4, recording the defects. By the method, full coverage for a detection part of the blade can be realized; the detection sensitivity is improved; the difficulty in ultrasonic signal identification of a part with a complicated structure in the blade is solved; frequent scanning of the conventional ultrasonic probe is eliminated; comprehensive detection of the blade can be realized by once scanning; the detection efficiency is improved; and the detection difficulty existing in the industry for many years is solved.