Abstract: The causes, positions and orientations of the cracks appear in the manufacturing of large diameter thick wall SAWH pipes are introduced.The characteristics of the ultrasonic echoes of different types of cracks are analyzed.The analysis indicates that for thick wall SAWH pipe, the welds tend to show crystal cracks and stress cracks.Crystal cracks, distributing relatively even a weld, can be easily determined by comparing the odd and the even waves on both sides of the weld.Stress cracks are smaller, so only by analyzing the ultrasonic waveforms of the primary wave and the secondary wave from both sides of the weld, and taking into consideration the reflection amplitudes of both sides, can the defects be accurately positioned and quantified, which puts high requirements on the testing operator.