The invention discloses an ultrasonic detecting method and device for a common thread tooth surface crack of a stay bolt. The device comprises an ultrasonic flaw detector, a small-angle transverse wave focusing probe and a reference block. The method comprises a step of processing the reference block according to the material and specification of a stay bolt to be detected and processing an artificial crack on the reference block, a step of uniformly applying a couplant on a coupling surface of the reference block, connecting the small-angle transverse wave focusing probe to the ultrasonic flaw detector and placing the small-angle transverse wave focusing probe on the coupling surface, a step of moving the probe such that a transverse wave sound beam is reflected on a thread tooth surfaceto generate a modified longitudinal wave and detecting an artificial crack of a part to be detected of the thread tooth surface at a relative position by using the modified longitudinal wave, and a step of performing ultrasonic detection on the stay bolt to be detected and judging the crack of the detection part of the stay bolt to be detected. According to the method and the device, the incidentdirection of an ultrasonic wave is changed by the wave pattern conversion, the effective coverage of the thread tooth surface crack is facilitated, the detection sensitivity is improved, and a small crack of the thread tooth surface which cannot be detected in the prior art can be effectively detected.