Abstract: In the B-scan image of the ultrasonic TOFD method, the diffraction wave image of defects appears in a parabola-like shape. In the coordinate system of the weld cross-section, the coordinates of the vertex of the parabola-like shape represent the position of the defect tip. When the inspected weld has a large reinforcement, the B-scan cannot be performed, thus failing to accurately locate the defect. To address this issue, a prediction model for the parabola-like image of defects is established based on the dynamic positional relationship between the ultrasonic probe and the defect end along the B-scan path. By collecting two A-signals from the B-scan inspection, the model is used to calculate the coordinates of the vertex of the parabola-like image of defects, thereby obtaining the position information of the defects. This forms a method for defect tip positioning based on the prediction of B-scan images in the ultrasonic TOFD method. The results of artificial and welding defect detection show that the proposed model and method can obtain the position information of defects in the weld cross-section using two A-signals, effectively overcoming the technical drawbacks caused by the limited accessibility of detection due to weld reinforcement.