Abstract: In order to achieve the automatic extraction of spatial location data for unilateral projection overlap defects in double T style weldments, the automatic discrimination method of unilateral projection overlap defects was put forward on the basis of original automatic discriminant criterion. Firstly, line by line search was carried out towards X-ray detection images of weldments left turning or right turning, and the unilateral projection overlap defects were found by using the method of the same distance from defect center point to the upper end of the image, then the depth location of overlap defects were judged by comparing the left or right projection distance of unilateral projection overlap defects, and the overlap defects were discriminated. The automatic discrimination of unilateral projection overlap defects in actual weldments was carried out, and the experimental results show that the provided method is feasible.