Abstract: On using the conventional ultrasonic reflection angle probe to detect automatic arc welding thick-walled narrow gap butt weld, it was found that there were some intermittent wave signals in the circumferential region and in certain depth interval, and the reflected waves being in horizontal position in the weld center. Ultrasonic TOFD inspection methods were employed for the detection analysis and it was found that the reflected wave signals were due to the dense crack defects in butt weld. The test results show that, ultrasonic TOFD testing combined with detection by ultrasonic method, can effectively detect dangerous defects of automatic arc welding thick-walled narrow gap butt weld.