Abstract: When using the longitudinal wave straight probe to detect the drill collar thread, because of the angle between the main ultrasonic beam and the inspected thread, the defects of the first two teeth at the large end of the external thread joint and the second three teeth at the small end of the internal thread joint are easy to be undetected. In order to prevent the defect from being undetected, this paper proposes to use a small angle probe to detect the drill collar thread. By calculating the incident angle of the small angle probe and designing and manufacturing the reference block, the effective detection of the bottom defect of the drill collar thread is realized.