Abstract: Described are the ultrasonic testing method and its theoretical basis with which N80 Q casing is detected for its longitudinal defects using a double crystal inclined probe at oil field exploration site. The inspection case conducted to Φ139.7 mm×9.17 mm N80 Q casing proves that using a double crystal inclined probe to detect steel pipes for their longitudinal defects can obtain higher efficiency with smooth operation and fluent inspection,and can better locate the defects in steel pipes as well.