The invention relates to an ultrasonic phased array imaging detection method for steam turbine blade root bolt holes. The detection method comprises the following steps that a first phased array probe is arranged on an inner arc side shoulder on the air inlet side of a blade root, and the air outlet side of the front fork tooth bolt hole and the air outlet side of the rear fork tooth bolt hole are detected; a second phased array probe is arranged on an outer arc front side shoulder on the air outlet side of the blade root, and the air inlet side of the front fork tooth bolt hole is detected; a third phased array probe is arranged on the outer arc rear side shoulder of the air outlet side of the blade root, and the air inlet side of the rear fork tooth bolt hole is detected. The ultrasonic phased array imaging detection method for the steam turbine blade root bolt holes has the advantages that the blade root of a steam turbine does not need to be demounted and mounted, and thus the assembling cost and the time cost of halt operation are saved while the detection precision is not affected. After parameters of a phased array detector are adjusted and stored, a detection result can be obtained without moving of the probes or with little moving of the probes after the phased array probes are attached to a workpiece, the detection efficiency is high, and time cost is saved.