Abstract: Under the action of long-term heavy load and alternating load, the idler shaft of the shearer is prone to cracks at the root of the shaft shoulder, which affects the integrity of the structure. In order to quantitatively analyze the root cracks of the idler shaft, a two-dimensional finite element model for ultrasonic testing of the idler shaft is established to clarify the propagation process of the ultrasonic wave in the idler shaft and the law of its interaction with defects. An ultrasonic testing experimental platform was built to verify the validity of the finite element numerical model. The time-domain waveform and spectrum amplitude changes when the shoulder echo overlaps with the crack echoes of different depths are analyzed. The results show that with the increase of the crack depth, the echo amplitude at the shaft shoulder increases; from the spectrogram It can be seen that as the depth of the crack at the root of the shaft shoulder increases, the echo bandwidth becomes wider, and the spectral amplitude increases with the increase of the crack depth. The least square method is used to fit the amplitude, and the fitting function of the spectrum amplitude and the crack depth is obtained to realize the quantitative analysis of the crack depth at the root of the shoulder.