Defect inspection of small-diameter metal cylinders is a difficult problem in the field of nondestructive testing. In this paper, the propagation characteristics of shear wave in a cylinder and the interaction mechanism with defects were investigated by the finite element method. The directional change of the shear wave is explored by extracting the shear wave signals at different moments. The effect of the defect radius of the circular hole on the propagation of the shear wave was investigated by B-scan. The influence of different positions and different lengths of defects on the propagation of shear wave was investigated by C-scan. Results show that: the main energy range of shear wave in the propagation process changes from 15°-36° to 20°-35°. The propagation path of an approximate equilateral triangle is proposed and verified. For round hole defects of different radii, radius can be determined by the amplitude and receiving time changes. When the defects enter the main energy range of the shear wave, the detection angle increases with the increase of the defect position. The position information can be determined by the detection angle of the reflected signal. With the increase of defect length, the transmitted wave ST energy decreases and the detection angle decreases, presenting an approximately linear energy distribution. Defect length information can be determined by detecting the angle of the transmitted wave reaching the outer surface. The propagation path and length calculation formula are put forward, and the error percentage of the calculation result is controlled within 6.5%.