Abstract: The effects of the center frequency of excitation signal, axial crack length, depth, and positions of defects on circumferential ultrasonic guided wave reflection were analyzed. Time-domain reflection coefficient (R) was used to evaluate the variation trend of the reflection strength with the defect size and the center frequency for axial crack. For a given size of non-through-wall axial crack, the R of circumferential shear horizontal (C-SH₀) mode increased slightly when the center frequency was increased from 55 kHz to 110 kHz. At a given center frequency, for through-wall axial crack, the R firstly increased linearly, then decreased, subsequently entered an oscillatory regime, and finally became stable with the increase in the crack length. Therefore, the conclusion of C-SH₀ mode had the same sensitivity to internal and external defects was given. The maximum and minimum values of R at varying axial extents were identified and can be used for defect sizing.