Abstract: While carrying on ultrasonic testing, the probe should be perpendicular to welding and the radio of urvature of testing surface is changing. According to the conventional method, the operator is easy to make fault judgment. This article establishes a mathematical model of defect location and an auto defection location with matlab. The above-mentioned an veduce the possibility of repairing work caused by misjudgment and provide ground for repairing thg fault component.